I came across this completely by accident.  I was dragging a folder up to the Recycle bin, which I normally keep in the upper left hand corner of the screen (not sure why I was dragging as I usually just use the Del key...), but I must have had my mouse all the way up against the edge of my screen when I let go.  A new toolbar docked to the top of the screen appeared!  The contents of the folder were displayed.  I could close the toolbar by right clicking and selecting close, or by turning the toolbar off by right-clicking and deselecting it in the Toolbars content menu. 

Turns out you can do this in Windows XP as well as Vista.  I wonder when this feature was introduced.  I've never seen it before or quite know what I'd do with it.  Interesting all the same.

I created a screencast of this and I've put it up on Silverlight Streaming beta.

I posted this to the Silverlight Streaming site partially because I wanted to see how that worked (dead easy for videos) and partially because Screencast is now a pay site.  If you are interested in Silverlight Streaming site you can host videos and Silverlight applications on it (currently for free, but look for ad-based free accounts or pay ad-free accounts coming sometime).  More info can be found here.

I should mention that I recorded this with Camtasia from TechSmith.  Also dead simple.