Someone already mentioned my favorite, Sharon Den Adel from Within Temptation, but I'm wondering if anyone else likes Lana Lane. I think she has an amazing voice.
Christina, Lacuna Coil
Johanna DePierre, Amaran
Sfinx, Ram-Zet
Marina Zrilic, Ashes You Leave
I'm also quite fond of the woman's voice in Antimatter. (Not really metal, though)
Ehh, just off the top of my head. I'll add some more later if I think of it.
edit: Stefanie Duchêne formally of Flowing Tears as well.
Yeah I've heard Lana Lane before but I didn't really know she did metal. Is it power metal? Cause thats what I imagine it would be judging from her voice. I only heard stuff from her covers album and it wasn't bad.