I have to concur with the Cristina Scabbia mention, as well as the Natalie Imbruglia
Something to mention though: Angela Gossow (Arch Enemy frontwoman) is INCREDIBLY hit or miss. She seems to fluctuate between ZOMG HOT and holy fucking shit is that the same person?
Example:

Angela on one of her "not hot" days.
Anyway, why no mention of Liv Kristine:

Although, I must admit, I think I am more attracted to her voice? I dunno, Ill let you know when I see Leaves Eyes w/ Amorphis next month
The bassist from the band Chthonic:

Something about a chick in corpse paint...
Also, the excessively cute japanese chick from the movie Wasabi (Who is apparently a pop star in Japan):
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Lastly, I seem to remember Mia Ståhl formerly of Project Hate being very attractive in the looks department, but I cant find any pictures of her (and GIS seems to think that Project Hate's newer vocalist Joanna is Mia, or something).
Anyway, there are a lot more, but I think Ill leave it at those for the time being.