Well well Valve remade Steams UI entirely and I must say they did a very impressive job!
I love the new features and the modern look, reminds me of the Zune UI

For screenshots and a complete feature list, look here:
http://store.steampowered.com/uiupdate/

To try the Beta, simply go to Setting -» Account in Steam and Opt-In for the Beta.
Though a word of caution, some people said the Beta UI caused some of their games to automatically redownload and the new Steam In-Game Overlay isn't that stable yet so be careful!