One of the first things I noticed about Vista was that they'd included thumbnails on the alt-tab dialog and on the taskbar buttons.  Thumbnails in alt-tab is nothing new.  I've been using TaskSwitchXP for a while now.  What is new is the thumbnails on the taskbar buttons. When you point at a program's button in your taskbar, you get a tooltip with an embedded thumnail.

As if that weren't cool enough, the thumbnail is actually updated in real time.  Observe here, I'm viewing Weird Al's White and Nerdy* and the thumbnail is updating at the same 30 fps as the actual video player (I added the red box for emphasis)!  Well, maybe not the same framerate, but it has to be pretty close.  How cool is that???

* Yes, I'm an unabashed fan of Weird Al.  He's like the patron saint of geeks.  If you don't like it, suck it.