There are so many you can't count them. Many made a small living putting out a few albums and singing at coffee houses. Here's a few of my favorites that are undervalued:
Bonnie Koloc - when she was a young a voice that could soar from one note to another effortlessly.
Priscilla Herdman - a voice like an angel. Her version of 'And The Band Played Waltzing Matilda' an anti war song is chilling due to the contrast of the words and her angelic voice.
Jo Ann Kelly - the best white blues singer I've heard. She actually sounds black.
June Tabor - awesome sense of timing
Mary McCaslin - small voice that just grows on you. Wrote songs that bare her soul.
Sandy Denny - well known but I believe under rated both as a brilliant singer and song writer.