Most unusual/mesmerizing female voice.

Natalie Merchant gets my #1 vote!
Yoko Ono has a very unusual voice but John Lennon seemed to be the only person that was mesmerized by it!!
Sia Furler, Joan Armatrading.
Well, this may be a stretch but Antony Hegarty from Antony and the Johnsons comes to mind. I would argue that it is a female voice though not necessarily from a female. Joanne Newsome fits the unusual but some may feel not so mesmerizing. One I think would have more universal appeal is Marisa Nadler. I would strongly recommend giving her music a try.
Bjork ... and she IS unusual too!
Margo Timmins of the Cowboy Junkies .
Oh Margo, sweet Margo...
agree, Margo . . .

Also, Hope Sandoval is up there
#1 for me: Dusty Springfield
#2 Annie Lennox
Hope Waits

Holly Cole
Jesse Sykes of Jesse Sykes and the Sweet Hereafter. Unusual AND Mesmerizing.
What about Barbara Streisand? Her new Village Vanguard release will keep your attention, or mesmerizing.
Joanna Newsom
Janis Joplin
Billie Holiday
Kudos for the Joanna Newsom pick. I throw Cat Power into the mix.
I like Beth Gibbons from Portishead, she captivates me with her voice.

I first thought of Joanna Newsome because she is unusual but her voice does not mesmerize, me it annoys me...

Cat Powers is a good pick too.
Dinah Washington, maybe not mesmerizing but definitely demanding of attention
Jane Relf . . . not necessarily that unusual (ie odd), but unusually beautiful and indeed mesmerizing.
Jsaah, Totally agree thanks for adding her
Kate Bush
Unusual? yup
Mesmerizing? see unusual
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Elizabeth, great call on Callas, was just going to add her glad you did.
Joan Sutherland early, early in her career what she could do with her voice, flexibility and strength, has yet to be equaled.
Yma Sumac, unusual, Dusty and Carly Simon mesmerizing plus Janis Joplin for being Janis Joplin.
Seconding Dodgealum on Jesse Sykes. Definitely some unnerving stuff on "Reckless Burning." Julie Miller on "Broken Things" a bit off the path too.

And yes, we do love Margo. :)

Billy Holiday in her prime.

I always liked the chick from Sixpence None The Richer also.

Beth Gibbons (Portishead) is another.

And lets not forget Eartha Kitt
Nina Simone, vastly underrated
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.

-- 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
There is some very good talent mentioned.I would have to say Nancy Wilson is one woman,,and there is a few others that have already been mentioned, that always pull me very deep into the song!
I'm going with Bjork. Love that voice!
Forgot the Queen...

Mary Fahl (formerly of October Project)
One of my favorite, Brandi Carlile. and let not forget the late Eva Cassidy.
No one has mentioned Lisa Gerard (Dead Can Dance).
Alison Moyet, Yaz and solo...
I am a jazz listener who keeps his palette clean with everything else 10% of time.But I would have to say that mesmerizing and therefore unusual would go to Beth Orton.So many fine voices to choose from but her music and voice combine to be almost trance inducing.You here many voices with pleasant quality or pitch perfect but for me Orton though very musically different hits you the way that Nick Drake does as a male singer.
Uhhh...Elizabeth Fraser.
I'll add Marti Jones (formerly of Color Me Gone) and Maddy Pryor, the contralto(?) voice from Steeleye Span.


Steeleye Span can be very good

signed: Little Sir Hugh

Caitlin Cary
Cheri Knight
Kelly Hogan
Carla Bozulich
Paula Frazer
Holly Cole
Johnette Napolitano (Concrete Blonde)
How about Linda Perry of 4 Non Blondes fame?