Velocity Raptor: a simple physics Flash game where the physics simulates special relativity. Lorenz contraction, time dilation, red shifts, visual distortions ... people seem to get stuck on level 30, though Gwern made it to level 31. It's one thing to look at equations, it's another to get a feel for it. I strongly recommend this to everyone.
Was fun. Difficulty falls off after level 34, so keep trying :)
I wish that at level 35+ the color of the background tiles doppler shifted. Thus if you were going too fast you wouldn't be able to see >:D
If it's worth saying, but not worth its own post, even in Discussion, it goes here.