Margaret Sanger retired as a nurse to stand up against the Comstock Act of 1873 (forbade the knowledge, use, and distribution of birth control) in 1912. She wrote articles, smuggled the diaphragm into the United States from Holland, opened up the first family planning clinic, emphasized in helping the African-American community, and a whole lot more. She was arrested several times for her "crimes", but she never gave up on what she thought was an important right for women.
Some great sites to learn more about Margaret Sanger's life and her contributions for women's rights to birth control are:
Planned Parenthood (I highly recommend the read, especially since it discusses published statements about Margaret Sanger that misquote or attempt to distort her image.)
About.com (A short, but sweet summary)
Everything2.com (There's a quoted portion from Margaret Sanger's autobiography here)