The Music Genome Project is the greatest thing in the history of everything.

[A Black Falcon]

Well, it's quite lacking in classical, European techno, and Japanese pop artists, which is what I searched for. :) I don't listen to much of anything in the way of general-release American music... of course, I'm not a music person, so that's not saying a lot (I can't just sit and listen to music, it's not interesting, as I've said before; I'd rather have silence or the TV (news, cartoons, whatever) on in the background; I do not buy music CDs, ever; etc), but it's true. I didn't even bother searching for the kind of music I listen to most often, of course. There's no way they'd have videogame soundtracks...

Looking through my music folder for things that actually have listed artists...

Examples of things I searched for that I didn't find (covering the previously mentioned categories, mostly):
Bizet
Bach
Vivaldi
Megumi Hayashibara
Roots and Leaves
Chance Thomas
Pete Ruskay
Mark Morgan
Brad Venable
Boa
Blümchen
MKL

Ones which I did find something for:
Beethoven (though it's playing jazz music, so I'm not so sure about the accuracy there...)
Ayumi Hamasaki
Yoko Kanno
The Ink Spots
Sasha
Underworld
Harvey Danger (not my thing, not sure why I've got that here...)

See what I mean? I know the problem: I'm not searching for the kind of artists it would list! Well yes, that's the point. As I said, I don't listen to most such music very much. :)