Best music-rhythm game on the internet.
http://osu.ppy.sh/
Basically, it's like the Elite Beat Agents game for the DS, but there are about 1,300+ ranked songs, and several different skins. The game is 100% customizable, and it's a blast to play. They have almost every type of music and they are always adding more. They usually add a few songs every other day or so. Plus, if you can figure out the editor, you can take any song you own and make a "beatmap" out of it. It's loads of fun, and I suggest you play it.
It has an online ranking system. I'm "finalrayen" rank #2375. Although that seems really far behind, considering they have had over 100,000 players, that's not a half bad rank.
