Time-Waster: Physics for Fun!

The above link is a game in which you must "fire" an atom in such a way that it doesn't collide with any of the (increasingly numerous) gravitational masses long enough to advance a level. It's very simple physics, really, but the computations necessary to make the movements realistic are a testament to the immense quantity of spare computing power we have at our disposal. This is kind of educational, really, if you need a quick intuitive brush-up on gravity and motion. I like this kind of thing: simple, easy to get better at, and reality-based. I don't want to get into on-line games.... again.