http://www.scienceyear.com/wired/ind...age=/planet10/
It lets you create your own world in 3D and even determine its characteristics. It's part of a minigame where you determine everything from orbits to plant life, and the success of your planet is based on the traits you choose. Pretty cool...I haven't been able to make a successful weird planet yet, though. Comets hit it and other sundry stuff, wiping out all life.
It's Shockwave-based, so those of you without mad Photoshop skillz can just make a planet and take a screenshot.
zen