Pokémon Showdown is many things:
A web site you can use for Pokémon battling
A web API for the web site for Pokémon battling
A game server for hosting your own Pokémon Showdown community and game modes
Pokémon Showdown simulates singles, doubles and triples battles in all the games out so far (Generations 1 through 9).
PS has a built-in chat service. Join our main server to talk to us!
You can also visit the Pokémon Showdown forums for discussion and help.
If you'd like to contribute to programming and don't know where to start, feel free to check out Ideas for New Developers.
Pokémon Showdown's server is distributed under the terms of the MIT License.