Check out Rush Frisby’s  Flickr Wallpaper Rotator application, written using  .NET Framework 2.0

This tray application cycles through your desktop background to a photo from Flickr.  You can specify tags, user names with how often you want the background image to change.

I have mine set to feature pictures from the Flickr colorful tag every couple of hours.

Cool.

Edited: Updated ro fix broken link (01/02/08)