Steam has been working on their new UI for a few months now, I’ve been using it for a while, but it sounds like it’s finally going to be released. The new version has some really nice cosmetic updates, as well as some added functionality (such as a much loved/hated achievement system).
Valve has been busy lately, what with Steam on the Mac, Portal 2, and now this. If you can’t wait until Monday, you can download the beta now, it’s probably safe to assume it’s pretty close to the release version at this point. You can check out the details along with all the information on the new features here.