I love physics-based games. Love 'em. (This is why I'm so excited to go and pick up LittleBigPlanet in a few hours. Squee!) There are loads of them online, some no more than throwaway webtoys and some are games with a surprising depth. Splitter, however is a good balance. It has enough content to keep you playing for a while but it's not so fiendishly difficult that you give up after a few levels.
The premise is simple. You have to get your little smiley ball-thing to the exit, hopefully picking up stars along the way. To achieve this, you need to cut through (or split) the wooden blocks holding him up. Draw lines with your cursor to chop bits off. Easy peasy. Go.