Valve is opening up Portal 2 to the community today with the beta release of the game's authoring tools - the same tools Valve uses - a chance for you to get cracking on new maps, puzzles and more.
Specifically, the Portal 2 Authoring Tools lets players make single-player and co-op multiplayer maps, character skins and models, music, sound effects—basically everything you'd need to help make our copies of the game feel fresh and regularly updated with community-made content. PC gamers can grab the tools via Steam as of right now.
More information at the official site about what's included and how to get started.
Authoring Tools Beta Release [Official Portal 2 Site]