Now that Zack has made it possible to create 3D plasmoids I had to try writing one myself, too.
It renders a virtual Earth model on your desktop, with subtle rotate animation and a day/night cycle, using NASA's Blue Marble imagery. And it has a rather crappy atmosphere as well.
It requires a shader-capable videocard to work, so minimum requirements are GeForce FX or Radeon 9500 although you'll want to use a bit better card than those for any smoothness.
It doesn't have any useful functionality, but if you just want to waste your processing power, then it does a pretty good job ;-)
The code can be found in KDE's SVN in playground/base/plasma/applets/bluemarble/.