One of the great free tools I've found is Dreamscene, which essentially lets you have an animated or moving background on the desktop. It doesn't seem to have any noticeable resource hit either, although I'd turn it off for games.
Some of the dreamscenes are basically still images with effects added, while others are more sophisticated or live HD captures. There are also animations, and screen captures from games and game menus. I mean you could grab fraps, and record beautiful landscapes from games - like Far Cry 2 or Crysis.
I've made a short 2:30 video I just show a few of them. Please be nice, it's my first youtube video! :)
I've compressed a 500mb file to about a 50mb file, hope it looks okay in YouTube. Looks better in 720p fullscreen:http://www.youtube.com/watch?v=e3rOB0gIVJw
Instructions and linkshttp://www.door2windows.com/windows-7-dreamscene-activator-activate-dreamscene-in-windows-7/
Dreamscenes from Deviant Arthttp://browse.deviantart.com/customization/?q=dreamscene
Dreamscenes from Dreamscene.orghttp://dreamscene.org/
I've started customising my desktop, as I've gone years with just the standard interface. Now I need to find a suitable program to shuffle the wallpapers. Also I'm going to start on what I can do about the start menu, and the taskbar now!