A cool game. You have to be using last.fm to track your music history, but if you are on last.fm, it takes your top tracks and quizzes you on the lyrics from them -- "what song is this from, who's it by, and what's the next line?" I find it surprisingly compelling, I keep falling into the "ok just one more round" effect. I think I'm pretty decent at it, I can consistently score above 70 on my top 500 all-time tracks. (With all-time top 1000, it gets a bit trickier...)

(if you're not on last.fm -- it's a music-statistics-tracking site that works with any music service (even your local mp3 collection!) and has been around since 2002. I signed up for it a few years ago, it's fun to mess around and look at your stats.)