Eva Cassidy. (I havent read all the entries above and dont know if anyone mentioned her). Try her Live at Blues Alley and Eva by Heart. Honorable mention: Mary Black (not the last 3 cds, go for her earlier stuff), Gilliam Welch (Revival mentioned above), Etta James, Irma Thomas. Iris Dement and Lucinda Williams belong in a class of their own because they are singer-songwriters and, well, because there's non one else like them.