Tofu Hut has published a huge list of blogs that have MP3s, arranged by genre. eg.,

BLUES/FOLK/OLDTIMEY music and sounds: American Memory Audio, American Music, Cantaria, Honey Where You Been?, Honking Duck, The Ragtime Ephemeralist, Tinfoil and Uncommon Folk.

http://tofuhut.blogspot.com/2005/05/even-when-you-dont-find-music-here-you.html