Need a brain break? Play at the sites below to train your brain. You'll need Flash Player and Java for the third one. The first is like the a GO vs. NO-GO test for ADHD - react quickly, but with no false starts (HT: intelligencetesting.blogspot.com), the second has a few logic games, but I especially liked the visual memory game with moving blocks (one of the new free games) - it makes the spatial memory part a bit harder. The third is not really a game, but provides animations between 2D and 3D configurations.
If you're a real math-guy or math-girl, this may be too ho-hum. You might check out Combinatorial Games - Demaine) instead.
Sheep Go-No Go
Logic + Visual Memory