Personally I don't like Otep or Arch Enemy, but if you're into shit like Nu-metal (Otep's basicallly Limp Bizkit with a female singer), or hardcore thrash, then Otep and Arch Enemy can be counted as decent bands.
A really cool Alt/Prog rock female fronted band is Stolen Babies. Chick's definitely got a unique voice and can scream her fuckin' ass off. If you like punk, there's a band from Wales called Brilliant Sins with a pretty kick ass chick singer. As far as traditional rock/pop goes, Milla Jovovich (commonly known as the actress in such movies as Resident Evil, The Fifth Element, etc, she was a singer before an actress) has one of the most amazing voices I've heard. Her sound is pretty classical but really up beat.
Well that's my ten cents, hope it provided some kind of help in your quest for some good female fronted bands.