Most unusual/mesmerizing female voice.

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.

I scanned the thread and was SHOCKED that I did not see a mention of EmmyLou Harris until today. Unusual, mesmerizing and just plain awesome! Plus highly respected by other musicians. She's had an incredible solo career, but has also appeared as background vocalist on probably hundreds of other records. Nashville Royalty and the Queen of Alt-Country.......

Sia

Her music is not for most people including me (some great songs though) but her voice is incredible.