Most unusual/mesmerizing female voice.

Yma Sumac.

Peruvian singer with a 6 - yes,that's SIX - octave range.

One of her albums is for sale right now.

I don't know about mezmerizing, but she is definitely unusual.

Anita O'Day

Melody Gardot, Joni Mitchell, Natalie Merchant

I'm glad this thread produced so much response. The older I get, the more I find myself listening to music with strong female vocals. This hobby never gets old!

Unique female voices always get my attention. Some modern finds.....

Yukimi Nagano of the Swedish band Little Dragon. A great sample here:
http://www.youtube.com/watch?v=tcP5ivpLbCM

She is just as good live. Little Dragon is great actually.

Yukimi lends her voice to a few other projects and jazz bands as well, including Koop and Sleep Walker

Another very unique and new to me performer, is Elsieanne Caplette of Elsiane. http://www.myspace.com/elsiane

Cool thread. I will check out the other suggestions.