Seems there's an open beta of a new Steam UI.
I'll post some screenies..
http://www.bastian-de-byl.de/newsteam_friends.jpg
How to get the Beta UI: Open up the main Steam window. Click View and navigate to Settings. Under the accounts tab there will be a Beta Participation section with a drop-down box saying there is one new available beta participation. The rest is for you to find out.
So far what I've noticed is that the UI is quite a bit faster, or so it seems. There are new sounds for notifications, which seem very similar to a certain sound effect from StarCraft. It looks a lot nicer then the old Steam, though there were Skins available for it. Invitations from other players to be friends now also shows up in your friends list. Navigating the UI seems a lot smoother now, yet I haven't fully tampered with all of it yet.