the "Summer Mapping Initiative" contest being held by Valve for Portal 2. A map called "Patent Pending" took the top prize by offering a wide mix of mechanics, another called "Infinifling" that's focused on "mid-air portaling techniques" ended up in second place. "Edifice," based in some of the destroyed Aperture chambers, picked up an honorable mention. All of the maps are free to download; just get the file, put it in your maps folder by following the instructions on the official blog and use the dev console to load it up and play.
Congrats to the winners. Valve's also hosting a Portal 2 music video contest for the song by The National included in the game. The deadline on that one is coming up in the middle of July, so you've still got time to film and upload it to YouTube. Good luck!