Most unusual/mesmerizing female voice.

Emilie Autumn.

Some great answers so far, especially Harriet Wheeler and Caitlin Cary.

Two favorites of mine, that I'm surprised nobody has mentioned:
1. Lhasa de Sela
2. "Lemonflower"...
http://www.youtube.com/user/coffeescup#p/u/36/pf0xIR6B364
http://www.youtube.com/user/coffeescup#p/u/28/-XkK9OJBI0c

Some great answers so far, especially Harriet Wheeler and Caitlin Cary.

Two favorites of mine, that I'm surprised nobody has mentioned:
1. Lhasa de Sela
2. "Lemonflower"...
www.youtube.com/watch?v=vgHePGGCatA
www.youtube.com/watch?v=pf0xIR6B364

I agree with Slaw-Natalie Merchant is very smooth. I'm going to her concert Wednesday. I'm glad to see that she is touring
again. Joni Mitchell (Court and Spark) is also great, IMO.

Jimmdavis - thanks for Lemonflower - I ordered her CD on CDBaby (backordered there) after listening to a few of her cuts on YouTube. Beautiful voice..just lovely. If you like her then definitely try Emily Barker - "Despite the Snow" is her CD ...just gorgeous! Here's a link for her song, "Nostalgia" (the video sucks ain't great, but it's the better of the versions for the audio that I could find). Oh, and Lhasa is a great recommendation too.