Orbiter is a space flight simulator game developed by Dr. Martin Schweiger. This game provides an accurate and realistic real-time simulation of spaceflight, from the Earth to the Moon and beyond. Players control a variety of spacecraft, including the Space Shuttle, the Apollo spacecraft, and a wide range of fictional and real-world spacecraft. Players can design and build their own spacecraft, or customize existing ones. They can also adjust the level of realism of their missions and scenarios, from simple to complex. Orbiter also allows players to explore the solar system, visit other planets, and even launch their own deep space missions. The game is highly modifiable, allowing players to create their own scenarios and content. Orbiter is available for Windows and Mac OS X and requires a 3D graphics card to run.
Space Engine
Spore
Kerbal Space Program
Universe Sandbox
Pioneer
Zen Koi
KSP, while cool in its own right, is not much like Orbiter. The two share the space setting, but the similarities end there. Orbiter is a full-fledged simulator, while KSP provides a more casual experience (good one!) For those who like space, spaceships, etc. (KSP also lets you create and upgrade your spaceship).