3D Universe Simulator
OpenUniverse (OU for short) is a fun, fast and free OpenGL space simulator. It currently focuses on the Solar System and lets you visit all of its planets, most major moons and a vast collection of smaller bodies in colorful, glorious and realtime 3D. If you've ever had a chance to visit Mercury or asteroid Geographos, here you'll find them looking exactly the same way, following exactly the same path as when you've left them.
Notice that OpenUniverse is not actively being maintained anymore and that users are recommended to use Celestia instead.