http://universesandbox.com/
http://store.steampowered.com/app/72200/
Found this some days ago as a sales item on Steam. For 7€, i just hit "Buy" and tried it out. Though it bugs on me, i think its a great tool to understand, how vast space is.
And you can do many fun things. Increasing the size of the sun, or moon, bombarding planets with asteroids (or other planets), placing new planetary bodies and watch how they would develop in fast forward, etc.
I recommend it to anybody with an interest in astronomical SCIENCE
