Go Ask Alice http://www.goaskalice.columbia.edu - this site sponsored by Columbia University's Health Promotion Program has authoritative answers to teen questions about physical and mental health.
SEX, ETC. http://www.sexetc.org - this site sponsored by the Rutgers University Center for Applied Psychology is similar to Go Ask Alice, but it also lets teens themselves get involved in the discussions about health & wellness issues.
Health Information for Teens http://www.fda.gov/oc/opacom/kids/html/7teens.htm - the United States Food and Drug Administration Department of Health and Human Services provides this info. They also host girlshealth.gov at http://www.4girls.gov , a site that focuses on young women's health.