Noob-proof (N)3DS "Custom Firmware"
Luma3DS is a program to patch the system software of (New) Nintendo (2)3DS handheld consoles "on the fly", adding features such as per-game language settings, debugging capabilities for developers, and removing restrictions enforced by Nintendo such as the region lock.
It also allows you to run unauthorized ("homebrew") content by removing signature checks. To use it, you will need a console capable of running homebrew software on the Arm9 processor.
Since v8.0, Luma3DS has its own in-game menu, triggerable by L+Down+Select (see the release notes).
Clone the repository with
git clone https://github.com/LumaTeam/Luma3DS.git
boot.firm is meant to be copied to the root of your SD card for usage with Boot9Strap.
This software is licensed under the terms of the GPLv3. You can find a copy of the license in the LICENSE.txt file.
Files in the GDB stub are instead triple-licensed as MIT or "GPLv2 or any later version", in which case it's specified in the file header.