I've been going nuts over AllOfMP3.
The pricing is ridiculously cheap, the selection is good. They have fewer total tracks than the iTunes Music Store, but they have some artists (Radiohead, The Beatles, Metallica, to name three) that iTMS doesn't have, and when they have an album, they have the whole album. Too many times I wanted to buy something from iTMS to find out that they only have maybe 8 out of 10 tracks available.
Of course, there's always the worry about legality. I did some checking before I starting using the service. This page seems to sum up the situation fairly well. Summation -- I expect that the RIAA and others are going to try to divert our attention away from the site, at least until they have a solution. I Am Not A Lawyer, but it sounds to me that at this point in time, it's okay. In the future, if they make the correct modifications to the law, this situation may change.
I've been using the service to fill out my collection with albums that I had on vinyl and never bought on CD, or to pick up hard-to-find albums that I've been searching for. I haven't been buying much that I don't already know, so I haven't been exploring much. But hearing some albums again after 20 years has been quite enjoyable. More often than not, I'll find that the music is still enjoyable, but the lyrics are execrable. On the other hand, sometimes I find out that I had better taste than I thought I did in High School. Well, everybody gets lucky sometimes.
Side note -- I was into metal in High School and early College. I
stopped looking into it right when underground metal (black metal, death
metal, speed metal, whatever you want to call it) starting growing. I
didn't stop listening to it because of that, I just moved on to other
things. But listening to the new Probot album gave me some appreciation
for that style of music. The Probot album rocks.