Cubi is a brainy portal experience designed for players who have played through most / all of Portal 2. The puzzles are primarily cerebral in nature, providing a challenge that the average Portal player will enjoy while still providing fresh puzzles for the more skilled player.
To run the map: Place the .bsp file in the steam/steamapps/common/portal2/portal2/maps directory, then launch Portal 2. Once in, open the console and type "map cubi". Enjoy!
Download links:
http://forums.thinkingwithportals.com/downloads.php?view=detail&df_id=857
http://myaperturelabs.com/file.php?id=713
http://www.moddb.com/games/portal-2/addons/cubi