Audiosurf is a game that allows you to choose any song ever (as long as it is on your computer) and maps it out as a track that you "surf" on. The track speeds up and slows down along with the tempo. The more intense parts are harder as well.
There are quite a few game modes separated by basically easy/medium/hard. My preferred mode would have to be the Mono modes because frankly, they're the least complex and the easiest to get used to.
The game is as hard or as easy as you want it to be. You choose the song and the mode. Ultimately, there are thousands of choices.
Audiosurf is easily one of the best deals in games ever. The sheer quantity of things you can do as long as you own a fair amount of music is astounding. Pick this game up if you enjoy music games at all.