Ingersoll Center offers a wide range of information and services to gender dysphoric clients such as information on therapists, support groups, lifestyle changes, and guidelines for pro- and post-surgical care.

http://www.psych.ucsb.edu/research/cep
This site from the Center for Evolutionary Psychology at the University of California, Santa Barbara, contains a paper describing evolutionary psychology.  A great deal of background information is also included.

Intersexuals
Information on intersexuality is often found in the "Diseases and Conditions" subcategory
under Health in Internet search engines (e.g.3 Yahoo). Several informative resources can be
found:

http://www.pbs.org/wgbh/nova/gender/

NOVA webpage on intersexuals.  Provided by Jamie Roth.

http://www.isna.org/

     Intersex Society of North America (ISNA)
     Includes recent news pieces, bibliographies, guides for parents of intersexed
     children, recommendations for treatment, support group information

http://www.ftm-intl.org/

     FtM International
     Web site for female to male transsexuals

Chromosomal Variations

http://www.genetic.org/
 

Link to homepage of the Klinefelter's Syndrome and Associates.  These are boys born with XXY.