An Interactive Space Simulator "Smash planets together, introduce rogue stars, and build new worlds from spinning discs of debris. Fire a moon into a planet or destroy everything you've created with a super massive black hole. You can simulate and interact with our solar system: the 8 planets,160+ moons, and hundereds of asteroids, the nearest 1000 stars to our Sun, and our local group of galaxies." [31Mb, Windows only, sorry, but see inside for similar Mac and Linux apps] [more inside]
Noctis is a free space simulation program/game written mostly in assembly by Alessandro Ghignola, an Italian programmer. It is downloadable for Windows and MS-DOS, but be warned there is quite a learning curve. It features a planet lander, onboard ship computer, a Fido Net style method of communicating newly named and discovered stars and systems with other users, and a haunting sense of being alone in an immense universe. Fan fiction. Screenshots reveal the outdated resolution of the program.
Next Best Thing to Being There. A Quicktime Mars Rover Simulation.
A computer aided simulation builds a spiral galaxy from its beginning. In all, 390,000 particles were placed in an arrangement similar to a newborn galaxy. The end result after three months is an event that is believed to take billions of years to occur. (animation)