Sleater Kinney - I remember the first time I heard "Call The Doctor", I couldn't believe what I was hearing, it seriously made an impact on me. When I had a band, they were one of my main influences.
Joan Jett - I couldn't name another female out there ever that so successfully fused Rock music with Punk Rock attitude. I saw her in concert once, back around '98, she was awesome.
Liz Phair - She can be flat out X-rated one second and completely bubblegum pop the next. She made Madonna's most contraversial material seem like nursery rhymes in comparison!
A couple of my favorite bands of all time feature female members, Kim Deal from the Pixies and Kim Gordon of Sonic Youth. To me, those 2 bands are both very important to the history of music & are very often underrated or just forgotten.
I don't think anybody here has mentioned the brit band "Lush". I really liked alot of what they did with their melodic style and beautiful simple lyrics. Check out the song "For Love" if you haven't already.