Go Ask Alice! - answers health related questions, including those on relationships; sexuality; and reproductive health. Produced by Columbia University's Health Education Program.
MEDLINEplus: Menstruation and Premenstrual Syndrome. - A service of the U.S. National Library of Medicine and the National Institutes of Health. Information on menstrual periods, PMS, cramps, and irregular cycles.
Menstruation at About.com - Information about menstrual cycles, including menarche, normal and abnormal mensturation.
Museum of Menstruation - MUM for short, this site is devoted to menstruation and selected topics of women's health, with a great deal from a historical perspective.