Think you can cook up more challenging Portal 2 test chambers than GLaDOS or Wheatley? You're in luck. Valve has released a beta set of authoring tools for the game that will let you create your own levels. The download is free for anyone who owns the PC version, and you can find it under the tools tab of the Steam client.
According to Valve, the authoring tools are the very same ones used to create the game. Enterprising modders will be able to make single-player or co-op maps in addition to adding their own skins, models, sound effects, and music. Valve will even hook you up with
Gareth Stephen Merchant and Vern J.K. Simmons if you want custom voice-over work. Just kidding.
Portal's puzzle-infused gameplay lends itself well to user-created levels, and I'm curious to see whether modders will embrace the game. Valve has already promised to release new test chambers as part of a free DLC package that's being targeted for this summer. Given its history with the mod community, I wouldn't be surprised to see Valve include the best user-created test chambers in future DLC releases—and poach their authors to work on the next chapter in the series. Thanks to Joystiq for the tip.