Have you seen all there is to see in Portal 2? Even the "Peer Review" DLC that was released for free? Good news, Valve is now planning the release of an in-game map editor for early 2012.
The editor is described as "easy to use," and will allow players to view, play, and vote on community created single-player and co-op test chambers with the click of a button. As of right now, Valve hasn't mentioned a specific platform for the map editor. PC is a no-brainer, but we'll have to see if they go with the consoles, as well.