Nobody's mentioned Buffy Sainte-Marie yet??
Also, Sandy listed Pat & Victoria Garvey----definately worth looking for!! One song they wrote/sang called "Trail of Tears" still hums around in my head from time to time..haunting! My dad used to produce folk concerts in our region and Pat & Vickie used to stay with us when they were playing close by-the late night Jam sessions were pretty cool!
Don't know if they count as folk or early country (blended together back then) but what about The Carter Family?