Steam 2
Having used Steam since it was a requirement for Half-Life 2 (and I had to download the thing on my 56kb modem), it really does seem to have fallen behind from a UI perspective. This is not to say that the others are great, but there is still a certain Windows Vista aspect to it that could do with an update.
Enter STEAM 2. Focused on:
A landing page that serves a greater purpose than just the store
Easily hop into into games with your friends
Encourage you to complete the infinite library of games we have all amassed over the past 20 years (see the progress bars)
Obviously the real steam is infinitely more complex, but I would love to see a revamp that focuses more on your existing library; filter by genre, progress, game suggestions from your own (large) library, AI suggestions for where you may have gotten stuck and more.
You can view the prototype here.