I find that they are along the same lines as Nightwish and found them around the same time. I love the same things about this band that I do about Nightwish, except that I find this band to be more macabre, more eerie and their usage of her voice is masterful!
The Gathering(I recommend the era with vocalist Anneke van Giersbergen)-they're atmospheric, poppy rock. Idk how to describe them. If you like The Gathering with Anneke, you would like her solo project Agua de Anneke and Paatos. [You must be registered and logged in to see this link.]
Amaran - They only had two albums before they broke up, but they're a metal band that plays in the style of I guess, Dark Tranquility and In Flames. I would youtube them. [You must be registered and logged in to see this link.]
Lacuna Coil - they're kinda generic to me. But if you like Disturbed you might like them. [You must be registered and logged in to see this link.]
There are tons of female-fronted bands especially in Europe, but I didn't really get into a lot of chick bands since they sing about more feminine things...but here's a bunch of recommendations from last.fm [You must be registered and logged in to see this link.]