NASA JPL-Caltech | 2011 Jan 06
Developed in conjunction with NASA’s Virtual Planetary Laboratory, the "Extreme Planet Makeover" on the NASA/JPL PlanetQuest site lets you roll up your sleeves and create your very own planet.
Balance five factors to create an Earth-like habitable world, or get wild and make your own extreme exoplanet. Use the Image Gallery feature to compare your creation with those of other Earthlings. Once you've finished creating the exoplanet of your dreams, download a picture of your custom world for posterity.
More information: Extreme Planet Makeover.