Additional content for Portal 2 (unofficial, but good)
From the website:
"Designed for Danger is a Portal 2 Campaign that offers eight new levels and 1 to 2 hours of play time. In the campaign, you assume the role of Chell during an early part of Portal 2. This time however, things turn out differently. The actions of a rogue force will set you on a new path through Aperture Laboratories."