One I haven't seen mentioned is Jacquie Naylor. "Live from the Plush Room" is a good place to start.
Second (or Third or 95th) the recommendations of Eva Cassidy "Live at Blues Alley," Cassandra Wilson (have four discs and all are great), Christy Baron, Patricia Barber (her cover of "Light My Fire" might be the five sexiest minutes of music I own), Madeleine Peyroux (esp. "Hey Sweet Man") and, of course, the great Ella (especially her duets with Louis Armstrong). Good listening.