From the official announcement on Steam:
Anyone else looking forward to this? Planning on messing around with the simple map editor? Do you know of great, balanced maps that are out already? I hope this increases the replayability of Portal 2 significantly. It was simply too short!Valve [...] announced the free Portal 2 DLC titled "Perpetual Testing Initiative" will be available for the PC and Mac on May 8th.
The "Perpetual Testing Initiative" allows players to easily create, share, and play Portal 2 puzzles. The Initiative comes with a simplified puzzle maker that allows that creation of mind-bending puzzles without ever leaving the game.
The puzzle maker can directly publish maps to the Steam Workshop where users can browse, vote on, and select to play them. Selected puzzles will automatically be downloaded and installed inside Portal 2.
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http://www.thinkwithportals.com