I stumbled on a really nice bit of software this morning called Gravity 3D (sorry, only runs on Microsoft Windows from what I can see). It's a galaxy collision simulator (and it's free). I can't say how "useful" it is in terms of the science but one thing I do know is it's very pretty and very addictive.
The Polygon Worlds site contains other astronomy related software too, including Mars Explorer, Mars 3D, Earth 3D and Venus 3D.
Be sure to have a look in the gallery too.