Valve's Steam service has gotten a revamp of its user interface but today every Steam user can now opt to try out the new Steam interface in beta form. As you can see from the above screenshot it's a pretty radical change from the current UI for the PC game download service.
Some of the new user interface features include improvements in the Steam friends list including ways to see what games they play and purchase and ways to invite them to the current game you are playing. There's also been improvements in getting news on all of Valve's library of PC game titles. You can now organize your Steam library of titles into custom categories. You can view Steam achievements in-game via the in-game overlay. There's a new version of the in-game web browser and lots of other additions, improvements and bug fixes.