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💲 = Paid, 🚩 = Limited Free, 🆓 = 100% Free, 🅾️ = Open Source

Please remember to always look at the license before using something in your project.

Assets

2D Assets

2D Cartoon Mobile Game UI Pack - cartoon user interface asset pack. It comes as a layered psd file. 🆓
420 Pixel Art Icons for RPGs - Set of 420 RPG icons, free for commercial use. 🆓
GameartGuppy - Cheap 2D Assets 💲
Games-Icons Set - free icons for your games. 🆓
Iconmonstr - Another free icons resource for your games. 🆓
Kenny Assets - Royalty free assets 🚩
OpenClipart - Collection of free cliparts. 🆓
Oryx Design Lab - Cheap high quality royalty free sprites 💲
SpriteLib - a collection of static and animated graphic objects (also commonly known as sprites). 🆓
Time Fantasy - Lovely SNES-Looking RPG Assets

3D Assets

Blender 3D models - 3D models, particle systems/effects 🆓
Matcaps - A Huge library of matcap textures in PNG and ZMT, organized by color. 🆓
Poly Pizza - Searchable library of low poly creative commons 3D models 🆓
Reiner's Tilesets - A blog with free 2D and 3D graphics. 🆓
3DModelsCC0 - Public domain 3D models 🆓

Audio Assets

GameSounds - Archive of all kinds of royalty-free game sounds. 🆓
FreePD - Public Domain Music 🆓
FreeSFX - free sound effects 🆓
Freesound - collaborative database of Creative Commons Licensed sounds 🆓
Kavex's GameSounds - Free sounds for any use 🆓
Musopen - Royalty free music. 🆓
Nameless: the Hackers RPG Soundtrack - high-quality by BoxCat Games 💲
Octave - free library of UI sounds, handmade for iOS. :free1:
PacDV - royalty free sounds collection. 🆓
SoundBible.com - Royalty-free of sound effects under various licenses. 🆓

Multiple Graphic Collections

CGTextures - A large collection of textures. 🚩
GameDev Market - a community-driven marketplace that connects indie game developers with talented asset creators. 💲
OpenGameArt - a media repository intended for use with free software game projects. 🆓
PlainTextures - Free high resolution textures, brushes and photos 🆓
Sketchfab - Publish & embed interactive 3D models. 🚩
Vecteezy - Free Vector Art. 🆓

Code

Ads and Monetization

AdMob by Google - Google's Ads and monetization service for mobile.
Appodeal - A programmatic ad mediation solution for mobile apps.
Unity Ads - Unity3D Official Ads SDK.

2D Engines and Frameworks

Allegro - Allegro 4 & 5 are cross-platform, open source, game programming libraries, primarily for C and C++ developers. 🅾️
Bladecoder - Classic point and click adventure game engine and editor. 🅾️
Box2D - A 2D Physics Engine for Games. 🅾️
Chipmunk C# - C# implementation of the Chipmunk2D lib. 🅾️
Chipmunk2D - A fast and lightweight 2D game physics library.
Cocos2D - graphic library for games and multimedia, for python language 🅾️
Cocos2d-x - a C++ OpenGL 2D and 3D game engine. Uses C++ but has JS and Lua bindings. 🆓
Construct 2 - an HTML5 game maker, meaning you are not actually writing JavaScript. Instead, you use actions, events and conditions to do the heavy lifting. 🚩
Coquette - A micro framework for JavaScript games. Handles collision detection, the game update loop, canvas rendering, and keyboard and mouse input.
Corona SDK - A Cross-Platform Mobile App Development for iOS and Android.
Defold 2D game engine by King 🆓
EasyRPG - role playing game creation tool compatible with RPG Maker 2000/2003 games 🆓
ENGi - A multi-platform 2D game library for Go. 🅾️
Ejecta - A Fast, Open Source JavaScript, Canvas & Audio Implementation for iOS. 🅾️
EnchantJS - A simple JavaScript framework for creating games and apps.
FlashPunk - free ActionScript 3 library designed for developing 2D Flash games.
Flixel - an open source game-making written in ActionScript3. 🅾️
GameMaker - 2D Game Engine 🚩
GameSalad - Game Creation Engine for Mac and Windows.
Gideros - Mobile Cross-Platform framework using Lua programming language. 🅾️
Glide Engine - Game engine for making 2d games on iOS, macOS and tvOS, with practical examples. 🅾️
Gosu - 2D game development library for Ruby and C++ 🅾️
HaxeFlixel - Create cross-platform games easier and free in Haxe.
HEAPS.io - A mature cross-platform graphics engine for high performance games written in Haxe.
iio.js - A javascript library that speeds the creation and deployment of HTML5 Canvas applications 🅾️
Jitter - a fast and lightweight physics engine written in C#.
Juno TypeScript - Clean and lightweight 2D game framework written in TypeScript
Kivent - A 2D game framework for Kivy.
Kivy - Cross platform Python framework for creating apps and games for Linux, Windows, OS X, Android and iOS
LibGDX - Powerful (totally free) library for Java, code once and run the game on desktop, Android, Web, and iOS. 🅾️
LimeJS - HTML5 game framework for building fast, native-experience games for all modern touchscreens and
Lums - A 2D / 3D framework written in C++11. Very efficient and modern. Still under heavy development. 🅾️
LÖVE - Lua 2D Game Engine. 🅾️
MINX - Open Source 2D game framework written in C++ (to the style of XNA) 🅾️
Matter.js - a 2D physics engine for the web.
MelonJS - open source light-weight HTML5 game engine. 🅾️
Monogame - Open Source implementation of the Microsoft XNA 4 Framework. 🅾️
NodeBox - a family of Python tools to create generative design.
Open Mega Engine
OpenFL - Open Source Haxe Engine for making multi-platform games. 🅾️
OpenRA - OpenRA is a Libre/Free Real Time Strategy Game Engine.
PICO-8 - A fantasy console for making, sharing and playing tiny games and other computer programs.
PandaJS - Open Source HTML5 Engine. 🅾️
Phaser - free and fast 2D game framework for making HTML5 games for desktop and mobile web browsers, supporting Canvas and WebGL rendering.
PixiJS - is a newcomer HTML5 game renderer - first released in early 2013. A main appeal of the engine is its use of WebGL for faster performance. If WebGL isn't supported, the engine falls back to standard canvas.
Processing - Processing is a programming language, development environment for artists, designers, researchers.
PuzzleScript - open-source HTML5 puzzle game engine.
PyGame - a 2D game engine in Python. 🆓
RPGMaker - series of programs for the development of role-playing games. 💲
Ren'Py - visual novel engine using the Python language in simplified form. It supports Windows, Mac OS X, Linux, Android and iOS 🅾️
SDL - SDL is a cross-platform library designed to provide low level access to audio, keyboard, mouse, joystick, and graphics hardware via OpenGL and Direct3D. 🆓
SFML - Simple and Fast Multimedia Library. 🆓
Solarus a free and open-source Action-RPG (Zelda) game engine 🆓
SpriteBuilder - Open Source Game Development Suite for MacOS 🅾️
SpriteKit - iOS/Mac 2D Game Engine.
Stage.js - Lightweight and fast 2D HTML5 rendering and layout engine for cross-platform game development.
Starling - The GPU powered 2D Flash API
Stencyl - a game creation platform that allows users to create 2D video games for computers, mobile devices, and the web.
Tilengine - C Engine with wrappers for C#, Python and Java 🅾️
Tiny Computer - a tiny computer where you can make, play and share tiny games.
vlc-unity - LibVLC plugin for Unity by VideoLAN to integrate multimedia playback inside your Unity apps and games.

3D Engines and Frameworks

Azul3D - A 3D engine written in Go.
bgfx - Cross-platform, graphics API agnostic, "Bring Your Own Engine/Framework" style rendering library. 🅾️
Bullet - Real-time physics simulation.
CRYENGINE - A pay what you want next gen 3D game engine written in C++. 🅾️
Cinder - Cinder is a community-developed, free and open source library for professional-quality creative coding in C++. 🅾️
Dash - A free and open 3D game engine written in D. 🅾️
Diligent Engine - A modern cross-platform low-level graphics library that supports Direct3D11, Direct3D12, OpenGL/GLES, and Vulkan. 🅾️
gameplay3d - 2D + 3D game framework written in C++ 🅾️
Godot - An advanced, feature-packed, multi-platform 2D and 3D open-source game engine. 🅾️
Horde3D - small open source 3D rendering engine. 🅾️
Irrlicht - open source high performance realtime 3D engine written in C++. 🅾️
jMonkeyEngine 3 - a 3D open-source game engine for adventurous Java developers.
JPCT - jPCT is a 3D engine for desktop Java and Google's Android.
Lumberyard - Amazon Lumberyard is a free AAA game engine deeply integrated
ODE - ODE is an open source, high performance library for simulating rigid body dynamics. 🅾️
Ogre3D - is a scene-oriented, real-time, flexible 3D rendering engine (as opposed to a game engine) written in C++.
OpenXRay - a community-modified X-Ray engine used in S.T.A.L.K.E.R. game series. 🚩
Panda3D - a framework for 3D rendering and game development for Python and C++ programs.
Piston - a modular open source game engine written in Rust. 🅾️
PlayCanvas - A WebGL Game Engine.
Polycode - Open-Source Framework for creating games in C++ (with Lua bindings).
Rajawali - Android OpenGL ES 2.0/3.0 Engine 🅾️
Source Valve's Flagship engine 🆓
Spring - A powerful free cross-platform RTS engine.
Stingray - 3D game engine and real-time rendering software 💲
Superpowers - HTML5 Collaborative 2D/3D Game Maker
Three.js - Javascript 3D Library.
Turbulenz - Turbulenz offers the ability to build, publish, iterate and monetise high-quality games that react like no others, with immersive 3D effects and real-time physics that open up a whole new world of unprecedented and extraordinary web content.
Unity 3D - A development engine for the creation of 2D and 3D games and interactive content.
Unreal Engine 4 - the new game engine technology developed by Epic Games.
Urho3D - Cross-platform rendering and game engine. 🅾️
Wave - Cross-platform engine written in C#
WhiteStorm.js - 3d javacript framework for building apps and games 🅾️
voxel.js - voxel.js is a collection of projects that make it easier than ever to create 3D voxel games like Minecraft all in the browser.
Xenko Game Engine - open-source C# game engine designed for the future of gaming 🅾️

Game Source Code

BrowserQuest 🅾️
Diablo 🅾️
Doom 3 🅾️
Doom 🅾️
Duke Nukem 3D: Atomic Edition 🆓
HandmadeHero: making 2D game from scratch 💲
Prince of Persia 🅾️
Quake 2 🅾️
Quake III Arena 🅾️
Quake 🅾️
SimCity 🅾️
Wolfenstein 3D 🅾️
Delver 🅾️

Modules and Additional API

AirConsole- Add controller support using broswer 🆓
ecs-lib - ecs-lib is a tiny and easy to use ECS (Entity Component System) library for game programming. It's written in Typescript but you can use on node.js and web browser too. 🅾️
kitlang - Kit is a programming language designed for creating concise, high performance cross-platform applications. 🅾️
p2.js - JavaScript 2D physics library 🅾️
SimpleAI - C++ AI behaviour tree based library with a QT5 based remote debugger 🅾️

Design

Idea Generators

Boardgamizer - Board Game Design Idea Generator. 🆓
Gigster Randomized Game Idea Generator 🆓
RPTools - a brand of open-source programs designed to enhance traditional pen-and-paper role playing games. 🆓
Video Game Name Generator Randomized video games names 🆓

Marketing and PR

Yunoia - Online Publishers/Marketing 🆓

Mechanics

Board Game Mechanics Set of different board game mechanics 🆓

Project Management

Asana - Free cloud project Management 🚩
Casual - Visual Project Management 💲
ClickUp - One app to replace them all. 🚩
Codecks - Project Management Tool inspired by Collectible Card Games 💲
HacknPlan - Project management for game developers 🚩
Taiga - Project management platform for agile developers & designers 🚩
Trello - Organize and prioritize projects 🚩

Tools

Animation

Fusion Character Animator - small tool for Clickteam Fusion 2.5 to facilitate the animation of 2D character sprites for developers. 💲
ScreenToGif - record a selected area of your screen and save it as a Gif 🅾️
Spine - Spine is dedicated to 2D animation, providing an efficient workflow both for creating amazing animation and for integrating it into your games. 💲
DragonBones - The Open Source 2D skeleton animation solution for Flash 🅾️
LWF - Lightweight SWF. LWF is an animation engine which can play animation data converted from FLASH contents in HTML5, Unity, Cocos2d-x, iOS UIKit, and more. 🅾️

Audio Tools

Ardour - A multichannel digital audio workstation 🅾️
Audacity - cross-platform software for recording and editing sounds. 🅾️
Audiosauna - transforms your web browser into a fast and flexible music production studio with built in synthesizers and live effects.
Audiotool - Online music producer.
BeepBox - online tool for sketching and sharing chiptune melodies 🆓
Bfxr - A tool to make sound effects for computer games.
Bosca Ceoil - Online (and also desktop) music producer made by Terry Cavanagh. Simple, intuitive, has a distinctive retro-ish sound. 🆓
ChipTone - Online sound effect generator 🆓
FamiTracker - Produce music for the NES/Famicom-systems 🆓
jfxr - A JavaScript port of the Bfxr sound effect generator.
LMMS - digital audio workstation software 🅾️
MadTracker - a powerful and efficient approach to making music. Versatility and compatibility are guaranteed due to full VST™, ASIO™ and ReWire™ support.
Musagi - fairly large and sophisticated music editor and synthesizer 🅾️
PulseBoy web-based musical sequencer which only uses 8bit sounds and samples 🆓
Rosegarden - music composition and editing environmen 🅾️
Sekaiju - Open Source MIDI sequencer. 🅾️
Soundation - Online Professional music studio.
SunVox - a small, fast and powerful modular synthesizer with pattern-based sequencer (tracker).

Bitmap Compression

ImageAlpha — Mac OS X GUI for pngquant and other tools 🆓
Image Asset Generation Photoshop plug-in — Easy export layers to files. 🅾️
Kraken.io - robust, ultra-fast image optimizer and compressor with best-in-class algorithms. 💲
PNGGauntlet - Smash PNGs for faster sites 🆓
PNGoo - Windows GUI for batch conversion. 🆓
Pngyu - simple PNG image file compression tool. 🅾️
SuperPNG Photoshop plug-in — Mac and Windows. Comparison with "Save for Web" 🆓
TinyPNG - Advanced lossy compression for PNG images that preserves full alpha transparency. 🆓

Character Generators

Charas - Charas is a charset generator for RPG Maker. 🆓

Design Tools

articy:draft visual environment for the creation and organization of game content. 💲
Dundoc Game Design starter Template 🆓
Scrivener Helps concentrate on composing and structuring long and difficult documents 💲
Tiny Game Design Tool small, portable booklet created in order to help game designers. 🆓
Twine - Open-source tool for telling interactive, nonlinear stories. 🅾️

IDE and Editors

Atom Highly customizable open source text editor 🅾️
Eclipse - integrated development environment mainly used for Java 🆓
Notepad++ Notepad replacement that supports several languages. 🅾️
Sublime Text Sophisticated text editor for code, markup and prose. 💲
Visual Studio Code Free. Built on open source. Runs everywhere. 🆓
Visual Studio Community Edition A fully-featured, extensible, free IDE 🆓

Modeling

Blender - free and open source 3D creation suite 🅾️
ZBrush Mini digital sculpting software 🆓
SketchUp - 3D modeling computer program for a wide range of drawing application 💲
Maya Industry animation software 💲
3ds Max Industry modeling software 💲
Clara.io - Start creating, rendering and sharing 3D 🚩

Pixel Editor

Aseprite - Animated sprite editor & pixel art tool 💲
Hexels - intuitive 2D, grid-based painting tool 💲
Pickle - Another Pixel art Editor. 💲
PiskelApp - Free Online Pixel Art and Animated Sprite Tool. 🅾️
Pixa.Pics - Load any image on the web app as a pixel art. Then, draw, and eventually vectorize art. 🆓
Pixi Paint Desktop version of Pixie Engine 🆓
Pixie Engine - Free online pixel editor 🆓
Pixen - Pixel Art Editor for OSX (Warning: Pixen previously did not support custom gif palettes before becoming paid, and may still have this limiation.) 💲
PyxelEdit - Pixel art editor designed to make it fun and easy to make tilesets, levels and animations. 💲

Spritesheet

Cheetah-Texture-Packer - High efficient and fast 2D bin packing tool 🅾️
GlueIT Free Simple Spritesheet Tool 🆓
Leshy SpriteSheet Tool - HTML5 tool for creating, packing, and modifying sprite sheets and texture atlases 🆓
Libgdx Texture Packer - Texture Packer built into Libgdx 🅾️
MakeAtlas - Simple tool to pack a texture atlas 🆓
ShoeBox - Adobe Air based app with game and ui related tools. 🆓
SpriteUV2 - tool for creating optimal meshes for textures/sprites and packing them into an atlas. 🚩
Stitches An HTML5 sprite sheet generator 🅾️
TexturePacker - Great spritesheet creation editor. 💲

Story Design

RapidQuest Free Branching Quest design tool 🆓

Terrain Generators

Canyon Terrain Editor - Create quality, realistic terrain quickly and intuitively 🆓
Fracplanet - Fractal planet and terrain generator
World Machine - Procedural terrain creation, simulations of nature, and interactive editing :heavy_dollar_sign

Texturing

FilterForge - A plugin for Adobe Photoshop that allows you to build your own filters. 💲
PixPlant - Smart 3D texturing tool that creates high quality normal, displacement, specular maps and seamless textures from photos. 💲

Tile/Level Editors

OGMO Editor - generic level editor. 🅾️
Mappy - Free tilemap editor. 🆓
Tiled - free, easy to use and flexible tile map editor. 🆓

Vector/Image Editor

Gimp - GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. 🅾️
Krita - Sketching and painting, offering an end-to-end solution for creating digital painting files from scratch by masters. 🅾️
Paint.NET - Paint.NET is free image and photo editing software for PCs that run Windows. 🆓
Pixelmator - Full-featured image editing app for the Mac
Affinity Designer - Professional graphic design software, easy to use and fast 💲
Affinity Photo - Professional photo editing software 💲

Voxel Editors

goxel - Open source 3D voxel editor 🅾️
VoxelShop- Open source for modify and create voxel objects 🅾️

Bug Reporting

Instabug - Bug Reporting tool for Unity that enables beta users to send feedback in-app. 🚩

Tutorials

Game Development Tutorials

3D Game Shaders For Beginners - Step-by-step guide to real-time shading techniques. 🆓
Simple HTML5 Canvas Game - Simple HTML canvas RPG looking game 🆓
Udacity: HTML5 Game Development - HTML5 Game Development by Google 🆓

A selection of major game studios, publishers, etc. using GitHub:

More games on Github