Kathrine Switzer (ran the Boston Marathon, 19 April 1967)
On 19 April 1967, Kathy Switzer became the first woman to run the Boston Marathon as a "numbered" entry--women were not allowed to compete officially in the race in 1967 (women were first allowed to register and compete five years later).
Switzer registered for the race as "K. Switzer." Race officials tried to block Switzer and get her off the course--the photos are amazing!
Switzer finished the race.
You can read her account of the race and check up on her life and career at Kathrine Switzer: Marathon Woman.
(Roberta Gibb, "Bobbi," was the first woman to run the marathon, in 1966, when she jumped into the race. She was running again in 1967, though not with an official number.)
Update: Fifty years after her first run in the Boston Marathon, Kathy Switzer ran--and finished--the race in Boston. In 2017, almost half of those who ran were women: 14,112 of the 30,741 runners who started the race were female. Of the women who entered, 94.5% finished.
As for Kathy Switzer, she wore the same number, 261, she had in that 1967 race. Here she is, crossing the finish line (at age 70):
"It’s a phenomenal social revolution, and it has happened in my lifetime,” Switzer told Outside Magazine. “To be there in Boston to celebrate that moment, the place where it all began, is extremely gratifying and validating.”