Leo Kottke. He described his singing as "Like geese farts on a muggy day."
Jorma Kaukonen.
John Prine.
Someone mentioned Leonard Cohen, but I think there was a big difference between his singing early in his career vs. later.
Somebody also mentioned Janis Joplin. Same thing - there was a tremendous difference in her voice between her early days (just a member of "Big Brother...") and when she was the headliner.