Another cool toy...

Welcome to Celestia... The free space simulation that lets you explore our universe in three dimensions. Celestia runs on Windows, Linux, and Mac OS X.

Unlike most planetarium software, Celestia doesn't confine you to the surface of the Earth. You can travel throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy.

It's similar to Stellarium, which Jello mentioned back in September.

EDIT: ...and apparently, possibly noteworthy linked this site back in May (before my time). Still, it's worth mentioning again. :)

Celestia