Cara Dillon I would recommend, also the Indigo Girls, The Wailin’ Jennies, Patty Griffin, and the wonderful Kris Delmhorst. A lovely, warm voice, great songwriter, and a lovely person as well. Patty’s touring with Robert Plant’s current band, and has a fabulous powerful voice, works with Emmylou Harris regularly too. Shawn Colvin I highly recommend, again a lovely voice and wonderful songwriter, great live, if the chance to see her presents itself. She does a great line in covers, too, like Gnarls Berkley’s Crazy. It wasn’t until I saw her sing it live with just an acoustic guitar that I realised just how damned good that song is.
Everyone else I would have suggested have already been listed above?