If you're one of the type of people who like to make game maps, then you'll enjoy this. Valve released the Portal 2 authoring tools, which are the same tools they used to make Portal 2. What's included:
"They'll allow you to create your own singleplayer and co-op maps, new character skins, 3D models, sound effects, and music." Sadly, this is still in beta, which means there might be a few bugs. Check out this blog post for links of mailing lists to join.