JOANNA NEWSOM
Her album Ys is beyond amazing and stands as probably my favourite album of all time. Especially if you have a soft spot for lyricism; she's unmatched. Honestly, if you can stick this album out, you're in for a real treat, its so densely layered, poetic and beautiful. The orchestration from Van Dyke Parks is mind blowing. I cannot stress the amazingness of this album enough.
A couple of my favourites:
Kimbra - Vows. Incredibly diverse Proggy Pop stuff. Try and get the 'original' version of the album, the NZ/Aus release, rather than the US, its much better.
Neko Case - Middle Cyclone. Sorta Rock/Country/Alt Pop.
Kaki King - Dreaming of Revenge. Awesome guitarist/vocalist.
Eisley - Room Noises. Dreamy bubbleguppie pop, amazing harmonies.
Battle of Mice - A Day of Nights. Awesomely disturbing Post/Doom Metal, with Lloyd Christmas' sister Julie on vocals.
B52's - Cosmic Thing. Classic Pop, the female harmonies on this album are second to none, incredibly tight if you consider that these were the days before pro-tools and pitch correction. 'Topaz' is a gem (lol)
All of these have probably been mentioned already lol but gahdam acquire the fire of these here albums. I listen to loads more female artists but you probably have enough suggestions by now and won't even end up listening to half the stuff you've been recommended lol.