Well, these are mostly musicians, not necessarily singers, but:

the original Carters
Arlo Guthrie
Norman Blake
David Bromberg
. . . a guy named Bob White who put out his own record 25+ years ago
Paul Geramia
Dwight Lamb
Kate Wolf
Bob Dylan
the Beers Family
Sam Hinton
Elizabeth Cotten
Etta Baker
Steve Earle
Swallowtail
Hazel Dickens
Robert Johnson
Phyllis Boyens
Creedence Clearwater Revival (OK, it's a stretch)
D.L. Menard
the Lost Bayou Ramblers (don't forget the Cajuns!)
Scott Biram
Woody Guthrie
the Red Clay Ramblers
the Freighthoppers
Allen Street String Band
High Woods String Band
. . . all those other great little old-time revival bands
Ramblin' Jack Elliott
Mance Lipscomb
Mississippi John Hurt
Brownie McGhee
Tom Paxton
Townes Van Zandt

and a friend of mine named Lane who passed away recently. He wrote his own novelty songs and played on a homemade folding guitar.