One of the most exciting features in Vista for me is Sideshow. You host these small gadgets on your Vista PC and it gets displayed on any device that supports Sideshow - laptop covers, cell phones, digital photo frames, remotes, etc. I think we're going to see a lot of innovation in this area with relatively little development effort.
Channel9 just posted a screencast that gives a quick overview of Sideshow and examples of how to develop gadgets for Sideshow. I can't wait to get a Sideshow-enabled photo frame in my house.