Most unusual/mesmerizing female voice.

For me its Sarah Vaughn. She's unusual and mesmerizing all at the same time. Many excellent tenor saxophone players wish they could play with the emotional impact that "Sassy" did.  

Patricia Barber??? Really? Here albums are DRENCHED in artificial reverb. I have heard that with the 180 gram version, they finally got the thing to sound decent.

Stacey Kent. 
Very seductive. It's like if Grace Kelly could sing.
Or maybe Kim Novak.

I haven't read all the nominations on this thread, so she may have already been mentioned, but Emmylou Harris has a distinctive and unusual voice. A pretty fair amount of vibrato, which I usually distain, but for some reason doesn't bother me in her case. Her voice has a lot of texture, being very "throaty", and she really inhabits the lyrics, giving them depth and meaning.

It’s like if Grace Kelly could sing.

Actually she does, and plays a mean sax also, but I think you referring to someone else :)

Second for Kelly Flint, who unfortunately has fallen out of sight.

And Stacey Kent is a favorite. Wish she was back in the states more.

Beautiful voice that has stood the test of time - Judy Collins. At 76 she still does 150 concerts a year.

Unusual voice - Nico of Velvet Underground fame.