Celestia

Roam the cosmos freely with Celestia, a free 3D space simulator. Celestia features a wide array of stars, galaxies, planets, moons, asteroids, comets, and spacecraft, with dynamic scaled viewing capabilities that let you see a cluster of stars, or a satellite only a few meters wide. A simple interface makes navigation to the furthest reaches of the galaxy a snap, while multiple add-ons can be downloaded and integrated later.