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.
Holy shit... after playing that for a while, whenever I quickly move my eyes the page I'm reading appears to stretch along the direction of the saccade and shrink along the perpendicular direction.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.