Following the Portal 2‘s first free “Peer Review” downloadable content, Valve will release a second add-on in early 2012 featuring an in-game map editor.
An “easy-to-use” editor, it will allow users to “design, build, and share” their own single-player and co-op test chambers. The online community can “view, play, and vote” on the maps with “a simple click.”
First screenshots of the editor can be seen below or at the gallery.
LittleBigPortal, anyone?