http://i.imgur.com/URO45.jpg
The Portal 2 Perpetual Testing Initiative is a really fun map editor and sharing platform. Making maps and sharing them with friends really brought me back to my early map making days with Wolfenstein, Doom, Warcraft 2 and Starcraft. Looks like Valve is taking the Perpetual Testing Initiative one step further and allowing people the ability to make co-op maps!

A snip from the Valve blog post:

"That's right: Puzzle creators can now design and publish co-op maps, which puzzle players can now greedily consume. Well, as long as they have a co-op partner. Which the 75% off coupon should help them (i.e.: you) find. We've also added a "Quick Play" feature that creates never-ending, auto-generated playlists of the top-rated maps in a variety of categories. It has literally never been easier to figuratively jump in and literally play some Portal"

http://www.thinkwithportals.com/blog.php?id=8663&p=1