Most unusual/mesmerizing female voice.

Dame Kiri Te Kanawa (more for the mesmerizing part of the question than the unusual part).

Kathleen Ferrier (both mesmerizing and unusual despite the ancient mono recording technology).

And a second for Elizabeth's mention of Emma Kirkby.

Regards,
-- Al

one last one from the late 60's Beverly Biven's of the We Five

Yeah, Audiofeil, Kate Bush without a doubt!

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!

Grace Slick
Harriet Wheeler (Sundays - Blind)
Bobby Gentry (Ode to Billy Joe)
Julie Driscoll (mesmerize me Baby)
Dianna Krall
Ricky Lee Jones
Dawn Upshaw