This is a compilation of the correct answers so far, I believe:

1. Joni Mitchel,l
2. The Almanac Singers
3. Greg Brown
4. Ben Harper
5. Judy Collins
6. Emmylou Harris
7. Harry Belafonte
8. James Taylor
9. Joan Baez
10.
11. Kingston Trio
12. Kris Kristofferson
13. Ramblin' Jack Elliot
14. Traci Chapman
15. Lucy Wainwright Roche