Space Engine - Home page
Space engine is a beatuiful piece of work. It is a 3D simulator of the entire universe. You can visit Mars, go to other galaxies, distant stars are just a click away. And if you are feeling super adventorous, you can go the the very edge of the universe. It really shows us how small you are in the huge universe.
You need a pretty good PC to run it, or a good laptop. When I played with it a few months ago is crashed often, so watch out for that. And if you're PC can take it, you can go to planet surfaces, other than our solarsystem, and it looks beautiful.
I highly reccommend this amazing masterpiece.