Bjork, Stevie Nicks, Beth Gibbons, PJ Harvey, Grace Slick, Bilinda Butcher, Patti Smith, Ann Willson, Janis Joplin, Kate Bush, Candice Night, Jon Anderson.

The chick from Cat Power has a pretty amazing voice

Cocteau Twins' singer can get grating after a while, but she's got a unique voice and style...I dig it.

I like Stereolab's female vocals, and The Slits, The Raincoats too obv. Patt Smith is a given, and Kim Deal/Gordon are good too (although Gordon has some bad moments).

and of course, Jon Anderson :)

Agreed. Tracey Thorn (Everything But The Girl) YouTube - EBTG(Everything But The Girl) - Rollercoaster(acoustic live)
Liz Fraser (Cocteau Twins) YouTube - This Mortal Coil - Song to the Siren "Cocteau Twins" are not two of the best lookers, but they have the voices of angels. My all time favourite vocalist is Maire Brennan from the Gaelic band Clannad, her voice is a gift from heaven YouTube - Clannad - Theme from "Harry's Game".

I also really rate:
The wilson sisters from Heart especially their 70's output.

Angela Gossow (Archenemy)
PJ harvey
The Girls of Sahara Hot Nights are pretty good
Jessika Fodera isn't too bad.
And my all time favorite is Pat Benatar

peaches <333

Yeah the girl from cat power has a ****king amazing voice. She caused a riot at Pitchfork

That explains a lot.

Kim Gordon
Polly Jean Harvey
Juanita Stein

the chick from the noisettes has the sexiest voice ever :)

patti smith
cat power lady
amy winehouse