For a little more brain challenging fun, I've discovered three mind-boggling on-line productivity killers:

1) The Trebuchet Challenge - set up the physics of your trebuchet to meet each of three different challenges.

2) Planarity - Just one instruction: "Arrange the vertices such that no edges overlap." It's sounds simple, and it is . . . at first.

3) Avoider - All you have to do in this game is to keep your mouse pointer safe. This is probably the most ingenious and original came to come along since Tetris!