[Azathoth]

I second Made Out of Babies, Julie Christmas' manic off-the-rails style contrasted with vulnerable croons is quite the experience. Check out her other project Battle of Mice as well.
 
Julie is also going to release a solo album later this year, watch out for that.

 

[batphink]

Another vote for Siouxsie.  She also had 2 other bands: The Creatures (with ex-husband Budgie) and The Glove (with Steve Severin and Robert Smith).
 

 
Shonen Knife
 

 
Meshell Ndegeocello (more funk and jazz than rock but still pretty good)
 

 

[Vincent01]

Anette Olzon...awesome.
 

 

[HarunoYume]

Haaaa, The Cranberries' "Zombie" is certainly one of my favourite rock song with female vocal. 

I saw a recommendation for Within Temptation, although I personally enjoy their music, I'm not sure it would be OP's style, but who knows.
Here's a completely different suggestion. (Not sure how you feel about non-english vocal/lyrics)

 

[Hank_Venture]

Probably not your style, but two who transcend the typical "Chick in a rock band"
 
Seasoned guitarists too.
 
 
Lori of Acid King
 

 
 
Wata of Boris (She hasn't done too many standalone songs)
 

 

[Choronzon]

Diamanda Galas (if you can find the album "The Sporting Life", which she did with John Paul Jones)
Jennifer Charles (of the band Elysian Fields)

 

[Crowe09]

Sharon den Adel (Within temptation)
Simone Simons (Epica)

 

[clc77]

Laura Veirs - I saw her on her last tour she was 7 months pregnant and rocked (in a mellow way
 

 
Sleater Kinney

 

 

[davidcotton]

Oldie but goodie check out All About Eve.  Should be able to find a "best of" reasonably cheaply now.  Oh and for a more prog type style check out Landmarq, in particular the last live album Turbulence.  If you don't mind a mix of male and female vocals then Phideaux albums "number 7" and "Doomsday Afternoon" should be heard.  Lana Lane might be another one worth checking out on the prog side.

 

[zumpp99]

Another vote for The Pretenders. Chrissie Hynde is one of the greatest woman rock singers EVER! Her vox are not of the soothing variety though (no hint of twee or sweet). She is a rocker who has perfected the tough/tender thingy. She is also a fiercely independent thinker if that matters at all

 

[lobotomizer]

Bif Naked
Die Mannequin

 

[Timestretch]

4 that I love:
 
The Cardigans
The Cranberries
Boa (not the korean one, but the Boa with the album "Twilight"
Curve