I've just remembered the other person I had filed away at the back of my mind with an Ada Lovelace Day 2010 tag.
Lise Meitner was born in Vienna in 1878 to a Jewish family. She studied physics and became the second women to obtain a doctoral degree from the University of Vienna. She moved to Berlin where she became Max Planck's assistant (the Max Planck of the Planck Constant but also the Max Planck who wouldn't, before Meitner, even allow women into his lectures).
Most of Meitner's research was in collaboration with the chemist Otto Hahn. In the early 1930s they worked on attempts to create elements heavier that uranium by bombarding heavy nuclei with neutrons. With the rise of Hitler in Germany and the Anschluss of Austria, it became unsafe for Meitner to remain in Germany and she fled to Stockholm. From Stockholm, she continued her correspondence with Otto Hahn, who by that point was getting some really interesting experimental results from his attempts to create heavy elements - what he found was that as he bombarded heavy nuclei with neutrons, the output was actually lighter elements.
It was Meitner, with her physics background, who provided the theoretical explanation for what Hahn was seeing in the lab. She suggested that, instead of sticking to the nucleus and making it heavier, the neutrons were actually splitting it into two smaller nuclei - the process we now know as neutron-induced fission.
As the practical implications of her work were recognised, Meitner was invited to join the Manhattan Project, which she declined. Otto Hahn received the Nobel Prize in chemistry for his work on nuclear fission, while Meitner was not recognised. In later life, she even refused to appear in a documentary about the atomic bomb as she did not want her name associated with it, though she is believed to have been bitter about not receiving any official credit and recognition for her work on fission.
Personally, I think she was an amazing woman and pioneer not just as female physicist but of physics in general.