Create levels for Portal 2

If you own the PC version of {Portal 2} you can head over to Steam today and pick up {Portal 2} Authoring Tools that let's you create new levels for singleplayer use, co-op including 3D models, sound effects, music and all the trimmings.

Take a look under "Tools" in Steam and you will find the program that Valve themselves used to create levels for the game.