http://www.spheresite.com/SphereXP **was** a desktop replacement for Windows XP only. It supports Vista and Windows 7 as of May 13th, 2009.
Basically what it does, is it turns your desktop into something like this:
Video of it in action:
http://www.youtube.com/watch?v=oeHe-li-cZE
I got it today, and I think I'll use this for a long time. My computer's running this just fine, even with my terrible specs.
Avg. Memory Usage: 20,000KB+, depending on what windows are on the desktop, and which ones are active.
Avg. CPU Usage [Single core P4 2.0GHz]: 10%, 3 windows inactive, 2 windows active refreshing at 200ms.
It comes with some skyboxes, and I think you can add more by making them yourself.
Skyboxes that can be used in SphereXP:
http://cubed.shadowpuppet.net/1.htmlIn XP, put them in
C:/Documents and Settings/<your account name>/Local Settings/Application Data/Sphere/SphereXP/Background [by default]
Get the .zip formats.