The new version of Steam has been in beta for a while now (I actually had forgotten that it was still a beta), but Valve have now stated that they'll be leaving the beta stage and pushing the update for all users on Monday. The new Steam looks a lot better, in my opinion of course, and is gives a better overview of your games, your friends, your achievements etc.
So if you haven't tried out the new version yet...well, come Monday you won't have much of a choice anymore. Learn to love it!