George M. Cohan: Tribute site includes biography, play summaries and reviews, and a list of stage productions, films, and recordings.
Musicals 101: George M. Cohan: Includes extensive biography and photographs.
TheatreHistory.com: Musical Comedy Is Born: Examines the early development of American musical comedy, specifically the contributions of George M. Cohan.
Wikipedia: George M. Cohan: Covers the composer's life, career, and musical legacy.