1. Vehicular Homicide
Olga Hepnarová, a Czech woman, deliberately hit a crowd of people with a truck, killing eight of the twenty that she struck.
Priscilla Joyce Ford, an African American woman, drove down a Reno sidewalk, killing six and injuring 23 more.
Brenda Ann Spencer, a 16-year-old, killed two adults and wounded a police officer and eight children at the Cleveland Elementary School.
Jennifer San Marco killed her neighbor, then drove to the Goleta postal plant where she had worked and killed six more people before killing herself.
Aileen Carol Wuornos, perhaps America's most famous female serial killer, killed at least seven men in two years.
Sylvia Seegrist took a rifle to a shopping mall, where she killed three people and wounded seven.
Amy Bishop killed three people and wounded three more at a meeting of the biology department where she worked.
Nasra Yussef Mohammed al-Enezi set fire to the tent where her ex-husband was being remarried. Fifty-seven people were killed, and ninety were wounded.
4. Angels of Mercy
Jane Toppan, a nurse, poisoned at least 31 people. Many of them were her patients.
Beverley Allitt, a nurse, killed four children and tried to kill more.
Kristen Gilbert, a nurse, was convicted of killing four people and trying to kill two more.
The list would be longer if I included family killers like Andrea Yates or women like Karla Homolka, Debra Denise Brown, and the Manson women who killed with men who were dominant in their relationship. But I must note that Tamerlan Tsarnaev was the dominant partner in the Boston bombings, yet few people say that's a reason to excuse his younger brother, Dzhokhar Tsarnaev.
My list relies heavily on A Brief History of Female Rampage Killers, but I've added a few.
ETA: I'm very fond of Wonderella, and I agree mass murderers should be denied their fifteen minutes of fame, but the latest comic repeats the notion that mass murderers are always male: The Non-Adventures of Wonderella » The SOME of All Fears.
ETA 2: Added the fourth category thanks to Ken Larson's suggestion in the comments.