Valve has officially announced an easy to use in-game map editor for Portal 2 that will let your users follow their imaginations to build and share their uniquely created single-player and co-op test chambers with community. I don’t know when it will be released but it will be early next year as the second Portal 2 DLC.
Valve will push some community features also with the second DLC. Notably, enabling users to vote for the best maps. Other than that, we have a glimpse of Portal 2 Map Editor that Valve sent us. Rest is all imagination!