There's been a number mentioned who I would +1:
Gemma Hayes, The Indigo Girls, Goldfrapp, Laura Marling among them, to which I'd add
The Wailin' Jennies, Canadian four-part harmonies to break your heart, (and lovely ladies as well),
Jesca Hoop, Eddi Reader, Julie Fowlis, Emmylou Harris, Patty Griffin, Shawn Colvin, Kris Delmhorst.
Kris has the most wonderful, warm, contralto voice that just wraps itself around you, and she's a fantastic songwriter as well, check out 'You're No Train', 'Free Diver' and 'Lullabye 101'.
Oh, and she's a lovely girl in person, too. Shawn and Emmylou and Patty are stunning singers, and tour together as well, at times. Then there's Beth Neilsen Chapman and Mary Chapin Carpenter as well.