Some good female players have already been mentioned - Kaki King, Maybelle Carter, Jennifer Batten. A few more:

#Gabriela, as in Rodrigo y Gabriela:A female Hetfield with an acoustic! I'd love to hear her plug into an electric through a Marshall and riff out.

#Emily J. Burton from Fireball Ministry: the band may just be Black Sabbath on steroids, but this girl can play.

#Adalita from Magic Dirt: Aussie band, I don't know whether you get their music overseas, but she can play quite well, and she's a great songwriter, too.

I agree with Aschleman and Earthman - guys win through sheer force of numbers. If playing the guitar wasn't socio-culturally such a "guy thing", I'm sure we would have just as many great female players as male ones.