Baseball History, http://www.where-at.com/sports/history.htm, A good compilation of baseball history links.
Baseball History, http://www.dmoz.org/Sports/Baseball/History.
Home of College Football, http://www.collegefootball.org, A good place to find information on how football became popular.
African-American Athletes in the 20s and 30s, http://www.chron.com/content/chronicle/sports/special/barriers/before.html, An essay on how African-American athletes made changes and began to break barriers in the 20s and 30s.
Women's Basketball Hall of Fame, http://www.wbhof.com/wbhof/whf_timeline.html, A good timeline of the history of women's basketball plus access to all fo the members of the Hall of Fame.
Sporting News Archives and Hall of Fame, http://www.sportingnews.com/archives/hof/, A good source of information on sports history.
ESPN's Century of Sports, http://espn.go.com/sportscentury, Another good source of sports history.