In approximate chronological order:

Hank Williams (the real one)
Homer and Jethro
Buddy Holly
The Kingston Trio
Bob Gibson
(the young) Joan Baez
The Clancy Bros. & Tommy Makem
Ian and Sylvia
Doc Watson
Dick McCormack (a local VT singer/songwriter)
Utah Phillips
Willie Nelson
Waylon Jennings
Jim Ringer
Ian Tyson (again, in his Cowboy Culture stage)
Tom Russell

Those are just the major ones that I can think of right away.

Peter