Chain Factor as an addicting little game, once you get your head around the rules. I'll have to confess, it took me a few tries before I fully grokked what was going on... but after that, I lost a good amount of time to it.

The basic premise is this: it's like Tetris, to some degree. The best summary I've seen is this: the circles disappear if the number of circles in their row or column is the same as the number in the circle.

Go ahead - give it a whirl, and see how it plays.

An interesting, and immensely nerdy sidenote: it's related to the TV show Numb3rs, and even has its own wiki. Now *that's* some fancy marketing for ya. [via]