Symphony, screenshot above, is a game that uses your own music library to generate enemies on the fly. Think of it like a shmup where the soundtrack (presumably) doesn't suck ass.
This video should illustrate what the gameplay is like. It's made by a company familiar with music games, but the reason we're interested is with the digital music part. How well it'll work is up to the developers, but it's something that we want to see more of.
In fact, games like this could help sell music in weird ways if users are posting that certain songs get them certain levels that are really great. A crap song could make for a really good level. At the very least, you'll be able to explore your music library in a weird way. [Joystiq]