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Alfonsina Morini Strada was the only woman ever to participate in the Giro d'Italia along with all the guys. The year: 1924. Alfonsina was as high as 41st place in the GC, in a race where the shortest stage was 250 km. Roads were unpaved, no cushy team support vehicles for the riders so they had to do their own repairs and take care of themselves.
A crash caused her to arrive outside the time limit on one stage so she was officially out of the race. But she was bringing so much attention to the race, that the officials allowed her (and some of the guys as well) to continue. She completed the Giro 38 hours back of first. Still, she made it. Many had abandoned during the race.
A more detailed history and other cool cycling history can be found at this blog.
A more recent standout in cycling history, Audrey McElmury, was the first US female to win the World Road Championships (Aug. 1969). A more complete listing of accomplishments of this Cycling Hall of Famer can be read here. There's also a nice "At the Back" article about her in the March 10 ('08) issue of VeloNews.
Nice bit of cycling history (since I'm tired of reading about the UCI/ASO/Pro Team arguments).