And this time it's original! I knocked it up in Java over about a day (though most of that was trying to work out how to turn "bouncing balls into each other" into something resembling a game).
It's called Chain Reaction. Basically, you have to bounce a certain number of balls into each other. The page (just like all my other applets) keeps track of your scores in a global high-score table, which I think greatly improves the competitiveness. And it's pretty addicitve (and hard) to boot!