Seconding the Emilie Autumn suggestion, along with the Lacuna Coil suggestion.
Quite a few symphonic metal bands have female vocalists as the "main" vocalist, though there's oftentimes a lot of male-voice death growling, but if anyone's interested: Epica, Within Temptation, Leaves' Eyes, Nightwish, Sirenia, Kamelot, and probably others that I'm forgetting...
The Birthday Massacre
Janelle Monae (she's got an amazing voice, IMO)
Loreena McKennitt
Yuki Kajiura's work (she's the composer, but her main vocalists are all women, and she writes really pretty harmonies, she's done mostly anime/game music- Kalafina; FictionJunction; FictionJunction YUUKA)
Android Lust (very...dark lyrics. and definitely not for everyone)
Universal Hall Pass
Inkkubus Sukkubus (has some male vocals, but predominantly female vocals, pagan rock)
Celtic Woman
Cherry Filter (Korean rock group, singer is a woman, haven't seen that many of their songs, but there are a few music videos on youtube)
Yeah I'm totally just tossing female artists at all of you and hoping that one of them will catch your fancy...some of them are more obscure than others