Looking for some games...

I'd go for Audiosurf, i am looking forward to it myself. Rocksmith looks decent too.

Something you could play is Punk-o-matic. It's pretty awesome! You can create your own music from a bunch of different pre-recorded tracks for 4 instruments (2 Guitars, 1 bass, and drums). I haven't played it in a long time, but it's pretty fun. I recommend the downloadable version, if you're going to give it a go (Go up to Games -> Punk-o-matic 2 PC). I guess now they're working on a new version that will be less of a game and more of an editor, which sounds pretty great.

Or, if you're in the mood for something that's just like Guitar Hero, you could play Frets on Fire.

I know that neither of these are really what you are looking for, but I thought I would just toss in some alternatives to what games everyone else will be suggesting. =P

The only one I know is Beat Hazard but the GTA games on PC let you import your own music and play them on your own radio channel.

Auditorium!!! NOWW

Super Hexagon it's cool too.

Get Audiosurf, IMO it's better than Beat Hazard.

You may want to keep an eye on Crypt of the NecroDancer

Other than that I don't know anything I could recommend. I play Stepmania a lot but that's probably not what you're looking for.

You know, I could never get into any of the Guitar Hero like games. I definitely plan to get Audiosurf whenever I do have the money. Auditorium looks like fun too. That Crypt of the NecroDancer looks interesting. I may have to keep my eye on that one.

1..2..3.. Kick It! is another Steam game that allows you to use your own music. It started out pretty bad when it was first released as early access, but the devs have improved it so much over the past couple of months.

You can play Bit Trip Runner with eyes closed if you memorize the beats.

the last thing i played which allowed custom music was GTA SA Gallery radio station,the memories..

The Bit.Trip games are cool. Try those

Beat Hazard.

You should like chime.

Sequence. How has nobody mentioned Sequence?! Ronald Jenkees ftw

Try Superbrothers: Sword & Sworcery EP. It is magical.