I've yet to catch the Sudoku (if you're not sure what it is, there's an explanation here) bug, though its devotees seem to find it fascinating. For them, then: Sudoku Combat, a site where you can play by yourself or against other people in puzzles rated from "Easy" to "Evil." The site calculates your win/loss ratio and generates rankings.
There's no money on the line, which is probably a good thing, because I think that Sudoku would be a prime target for computer bots using brute-force attacks.