Funnily enough, I was reminiscing about AudioGalaxy over lunch today. Probably my favourite (or at least most fondly remembered) music download service - I used to love being able to queue up a load of music (and there was loads of really rare stuff on there) and then leave the client running while you were out of the house (in the dial up days - couldn't tie up the home phone line for too long otherwise) and see what it had downloaded for you when you got back in - quite often some little gem you'd queued up months ago and forgotten all about, but the one user with it had finally come back online!
The only place I can think of which a comparable selection of rarities (at least for electronic music) is Soulseek, but AG seemed more reliable, and the split client/web concept was brilliant. I'd pay a decent amount for a legal version of it, if they could somehow get record labels to agree that any file (including white labels, bootlegs etc) could be shared and they'd just get a cut of whatever they could match up to a label/artist. Not holding my breath!