Antony, of Antony and the Johnsons. Though he's not female, his voice certainly qualifies. Many of my other favorites have already been named; Joanna Newsome, Margo Timmons, Nina Simone (sounds like Antony), Kate Bush, Tori Amos, Laurie Anderson, Carla Bruni, Diamanda Galas (personally a bit too much for me), Montserrat Figueras, Imogen Heap, Kate Rusby, Vienna Tang, Lhasa, Teresa Salgueiro (Madredeus), Karin Bergquist (Over the Rhine), Karen Peris (Innocence Mission), Frazey Ford (The Be Good Tanyas)....
I love female vocals...I could go on!
