As promised, Portal 2's "Perpetual Testing Initiative" DLC is now available for everyone that owns the game on Steam. This is a "simplified puzzle maker" that allows players to create, share, and play homemade Portal 2 puzzles via Steam Workshop.
Within an hour of release, there are nearly a hundred maps already created--and that number is likely to increase exponentially.
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In the future, Valve plans on expanding the creation tools to allow for co-op puzzles. Until then, aspiring puzzle makers will want to create the most fiendish single-player challenges.
If you don't already have Portal 2, Valve is running a limited time sale on the game. You can pick up Portal 2 for $6.79, or get a bundle of Portal 1 and 2 for $8.49.