WordNet (r) 3.0 (2006):

folk singer
    n 1: a singer of folk songs [syn: folk singer, jongleur,
         minstrel, poet-singer, troubadour]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "folk singer":
   ballad singer, balladeer, bard, fili, folk-rock singer, gleeman,
   jongleur, minnesinger, minstrel, rhapsode, rhapsodist, scop,
   serenader, street singer, strolling minstrel, troubadour,
   trovatore, wait, wandering minstrel