If you are interested in even semi-obscure music, the best thing out there that I'm aware of is still Soulseek. You can find almost anything there if you're patient.
Shareaza is the best client out there by far, at least for Windows. It's what I use to download anything that I think my wife would have heard of. But when I'm looking for something I heard once five years ago on a college station, Soulseek is where I go.