I can think of dozens of female singers, some with groups and some solo what about:

Anne Briggs
Frankie Armstrong
Peta Webb
June Tabour
Hilary Spencer (even in Artisan, she and and Jacey outnumber Brian)
Dangerous Curves
Peggy Seeger
Jackie & Bridie
Belle Stewart
Jeannie Robertson
Bonnie Shaljean (sometimes on Mudcat)
Eliza Carthy
Anni Fentiman
Norma Waterson
Julie Felix
Nadia Cattouse
Joan Baez
Isobel Sutherland

I could go on and on - as far as floor singers go I can think of clubs and sessions which often have more women than men, try "Travelling Folk" for instance. If you go to the sessions in the barn at Towersey, the ladies definitely are in abundance (both numerically and by volume - ducks missiles from JudeL, LTS, Pearl etc) - I've even known men to wear skirts on the Saturday night, just to get a song !