I know that this has been talked about and demonstrated a thousand times but if you haven't seen it, Aero Peek is very cool. Not only that, but I find it the new feature that I use the most right now. Aero peek gives you the ability to see the application window for a given app when you either alt-tab (and wait) or move the mouse over the task bar thumbnail. All other open windows just show up as frames. I love the fact that I can see things like IM windows, documents, mail, etc without having to actually tab to them or even open them.
The only issue that I had with it was that you need to Aero turned on and you need to make sure that you have the drivers installed for your video card. On the Dell M6400 that I'm using, I had to pull the Nvidia drivers down from Dell in order to turn on Aero. Also you may have to turn on the feature at the taskbar level. This is under the Properties page of the taskbar.