Designed For Danger is a Portal 2 mini-campaign created by Patrick Murphy, which adds eight new levels and around 1-2 hours of gameplay to Valve's first-person puzzler.
These aren't just random levels, Murphy has actually carved a little alternate reality storyline out here, which, considering the pack is free, should make it worth a look for Portal fans starved of new content.
You can download the campaign below.
Update - While we're on the subject, I'm being pointed towards 12 Angry Tests, which looks even better.
Designed For Danger [Site]