First, I am going to recommend a band that is NOT metal. My Bloody Valentine. Now, that said, I think they will definitely appeal to fans of such bands as any of those old gothic metal bands with female vocalists. They are characterized by a combination of loud abrasive, but hypnotic, guitars mixed with a dreamy, drugged out almost lush symphonic sound. It's quite breathtaking, and absolutely inspiring. Get their album Loveless, it's easily one of the best rock albums of the 90s. It was an instant classic, and the popular indie rock zine Pitchfork Media gave it the best album of the 90s award, and I come very close to agreeing, especially in the realm of female led rock.
That said, you should get these bands for more of the metal spectrum:
Maudlin of the Well
Now, they don't have female lead vocals, and they have a vast majority of male vocals, but when they have female vocals it is absolutely inspiring, especially in songs like Girl with a Watering Can and Blight of River Systems.
Also,
The Gathering
They have already been mentioned, but The Gathering's metal albums with Anneke (Mandylion and Nighttime Birds) are by far the best metal albums with lead female vocals, and their new material is also quite inspiring.
Also, I believe some of the Unholy album Gracefallen had female vocals, and that album is absolutely mandatory if you like doom metal at all. But, if you like doom metal, you probably already have it. Regardless, it is a twisted and nightmarish journey into a pure aural hell. Recommended without hesitation.