Relativity Visualized
spacetimetravel.org
spacetimetravel.org
The premise is that you control a character moving at relativistic speeds. The first handful of levels are just to get used to the controls, and not very interesting. It is worth sticking with it for the later levels.
Another more immersive but less "instructive" game is "A Slower Speed of Light": http://gamelab.mit.edu/games/a-slower-speed-of-light/ It's pretty cool, though it gets disorienting by the end.