Recommended female vocalists?

Liz Phair, Lisa Loeb - Firecracker (great 3d+ recording but mostly binkie music, alas), Basia, Billie Holiday

A few faves:
Patricia Barber
Margo Timmins
Diana Krall
Cassandra Wilson
Jane Monheit
Joni Mitchell
These are just off the top of my head and in no particular order.

My additions to a lot of good folks already mentioned:

Rebecca Gates, 'Ruby Series' more of an extended EP (7 songs) but well worth it
The Innocence Mission, 'glow'
Everything But the Girl, 'Walking Wounded,' electronic, but Tracey Thorn's voice is amazing

And though this isn't all female vocals, the soundtrack to the movie, 'First Love, Last Rites' has great collaborations between Shudder to Think and Liz Phair, Nina Persson (Cardigans), and Angela McCluskey (Wild Colonials).

You might not know these femail vocals, but for what I've seen listed the ones I will list to you are THE BEST:

Mireille Mathieu -- Une Histoire D'Amour hard to find on CD but vinyls are often on ebay and have excellent recording. Genre is Chanson.

Patricia Kaas -- Scene De Vie and/or Mademoiselle Chante Blues. Genre is classic jazz, blues.
available on CDs.

Juliette Greco -- Any album will be very hard to find since she was popular in the mid 50's to the end 60's. There are some phillips german or UK pressing on vinyl with excellent recording. I'm not sure if she ever was printed on CD.

3 different and extra-ordinary French singers which can capture your heart even if you do not understand French.

I've started to learn French since I am so devoted.

(By the way the same way I've started to learn English since Eric Clapton was my favorite bluesmanm -- one of the vast reasons!)

Lucinda Williams: smooth and sexy