Celestia is the coolest program ever.
Basically, it's a universe simulator of sorts. You start in the solar system. You can surf around, looking at objects in the solar system from all angles. Lock on this object, follow that, and you can have some really neat views.
For example, hit enter and type "Iapetus". Hit C to centre. Hit G to go. There's Iapetus. No big deal. Use your mouse scroll wheel to make Iapetus smaller. Click on Iapetus and drag it to the lower right corner of your screen. Right click and drag on Iapetus until you see Saturn in the image. (It takes a bit of looking. Saturn isn't huge from Iapetus, but it's definitely visible.) If you can't find Saturn, hit enter and type "Saturn" and then c to centre. Find Iapetus again, click on it, then right click-drag as above until Saturn is in the frame. I like to put Saturn at the top left. Once you have them positioned pleasingly, click on Saturn and select "follow". Click on Iapetus and select "sync orbit". Then speed up time (L if memory serves; K slows it down; J reverses it). Cool huh?
The Windows version is excellent. The Linux version seems good too, although Ubuntu and Debian package an old version. I compiled the current 1.4.1 version from source and after some mucking about, got it to work. I also had to install a new video driver on my desktop to get good performance. You need 3D video to work. Unfortunately I can't get 3D video to work on my notebook.
Very fun... and free. There are also expansions available. I installed the two million star package, plus Eta Carinae, which is quite likely the next star in our stellar neighbourhood to go supernova. (It's far enough away that it probably already has gone supernova, but it takes 10,000 years for light to reach us from this system.
Anyway, download and try this great program.