Many of my favorites are already listed like Rickie Lee Jones and Karen Carpenter. Forgive me if I missed it, but I'm surprised Eva Cassidy wasn't mentioned yet. Another singer IMO worth checking out is Shawn Colvin. I'm not too hip to the classical female singers but out the ones I am familiar with I'd have to give Renee Fleming and Anna Netrebko a nod.
Enjoy,
Tom
Enjoy,
Tom