Discovery Channel's Greatest American # 22: Rosa Parks

All she may have done was sit down, but doing so in the segregated South meant taking a stand for civil rights. Rosa Parks demanded to be treated with dignity, and in so doing, ignited a movement which would ensure that black Americans would be treated as Americans.

I'm not sure she merits inclusion within the 25 greatest Americans for this---she was more symbol than leader in the movement Martin Luther King, Jr drove. One can certainly understand why her example has so resonated down through the years, however.