Love it or hate it, it seems there's no getting away from what is generally described as "female-fronted melodic metal". In its early days I remember it being described as "goth metal", though quite why is not 100% clear to me. We all know how it really began: Nightwish. Regardless of squabbling over who may have been the first band to play in this style, it was Nightwish who really brought this sort of music into the public perception, and essentially defined it.
The name is fairly self-explanatory, which is the strength of this classification. The music which falls under this heading is metal - i.e. electric guitars playing "heavy" riffs (which excludes bands like Girlschool); it's melodic - i.e. strong melodies often backed up by keyboards & "clean" (non-screaming) vocals (excluding, for instance, Arch Enemy); and the main vocalist is female. Leading proponents at the moment are Nightwish, Within Temptation, Epica, Lacuna Coil, and After Forever - to name but a few with whom I happen to be familiar. While they are all linked by this shared heading of "female-fronted melodic metal", it should definitely be noted that there is a good degree of variety between them.
Until recently, I would have to have admitted to being disenfranchised with Nightwish; despite being a moderate fan from first hearing them many years past, I felt that toward the end of Tarja Turunen's reign they became a caricature of themselves and creativity descended into a disappointing formulaic performance. With the coming of Anette Olzon, the sound has changed away from the characteristic operatic vocals (now generally the sole domain of Epica), to a more generic vocal sound. However, this change has allowed them more creativity in the music itself, meaning that this new direction could well be just the boost they needed in order to stay fresh and interesting. While their new album Dark Passion Play contains many remnants of their past over-inflated pomposity on tracks such as "The Poet and the Pendulum", there is real potential for a promising change of direction in the horrifically catchy "Amaranth" and the vitriolic "Bye Bye Beautiful". Despite the latter being full of references to the soap-opera nonsense that has been Nightwish over the last few years, it's a passionate and entertaining track.
My personal favourite artist from the genre at the moment would have to be Within Temptation. I was captivated by their album Mother Earth (despite it containing only around 50% good songs), and while their follow-up offering The Silent Force failed to live up to the potential of its predecessor, their latest, The Heart of Everything, is just what the doctor ordered.
Despite departing from their Mother Earth roots in terms of style, and moving from that towards a "mainstream" sound, it's still distinctively Within Temptation, and it's still innovative and truly entertaining.
As for the others mentioned above, Epica are a good band, and well worth a listen - unless you dislike Tarja-era Nightwish. The sound is very similar, and despite a lot more in the way of innovation and experimentation in sound, they've never convincingly managed to step out of Nightwish's shadow.
Lacuna Coil are far different from the others mentioned here: there is none of the grand theatricality that marks Nightwish, Within Temptation and Epica; rather, the direction they have chosen is a more electronic sound, and while they fulfil the categories of the genre, they don't often get described under this heading. Perhaps this is because they wouldn't sound as different as the others would were the vocals replaced with male ones.
After Forever are another good band with little to recommend them over any other "female-fronted melodic metal" band. Their earlier music is a more interesting listen, being quite progressive: Invisible Circles was a concept album with some truly great tracks. Their newest, curiously eponymous offering is generally Nightwish-y, with a little electronics and a few growly vocals to remind you that you're not actually listening to the Finns but one of the clones. That said, tracks like "Transitory" are far heavier than anything Nightwish have released, so perhaps if they develop this sound it could prove to be their way of stepping out of that damned shadow.
As a genre, when it's done well, female-fronted melodic metal can be very effective indeed, but sadly it is full of bands which are seemingly trying to emulate the band seen as the best at this stlye: Nightwish. We'll have to see where all the bands go from where they are now, but as things stand I would have to say that Within Temptation are in the best position at the moment.