Celestia

The graphics are really amazing- probably the best I've seen in an open source application. And it runs on Windows/Mac/Linux (I've used the first two, so far...)!

I find the controls a little opaque, but I haven't spent a lot of time RTFM. My 5-year-old twins have recently developed an interest in space (they are upset at Michael Collins didn't get a turn on the Moon), so it's great fun to zoom around the solar system with them.

The Mac port seems a little unstable (it crashes from time to time), but frankly it's more stable than Google Earth has been for me.

This is an excellent software, great educational tool and a free alternative to Space Engine. The graphics are awesome, but some resources are missing in the pure free version (additional non-free resource package is available). There's not only the Solar System, but also other systems (look in the menus). Also included are small space bodies and spacecrafts (yes, you can watch the ISS fly around the Earth). The time can be manipulated (stopped/slowed down/sped up/reversed/synced with real time). Orbits, labels and info such as the temperature can be displayed. Things that should be improved to gain the extra rating star: installation/packages on some distros (had some problems), controls (a bit confusing and seem hardcoded), some missing textures/3D models among the free resources, some text errors in GUI.