General:
http://en.wikipedia.org/wiki/Transgender (seriously. It's useful.)
The Gender Trust: the largest UK charity supporting people whose lives are affected by gender identity issues.
http://www.gendertrust.org.uk/
GIRES, the Gender Identity Research and Education society. Mostly academic: it publishes literature, conducts research, trains organisations and advises policy makers, with the intention of improving the lives of trans people in society by generating supportive attitudes in people who can bring about improvements. It focuses particularly on people with atypical gender identity development, and is also an excellent resource for journal articles.
http://www.gires.org.uk/
T-Vox: advice, information and support for transgender and intersex people and their friends and relatives. http://t-vox.org/
Transsexuality and healthcare:
Transsexual UK: lots of resources and information. http://web.ukonline.co.uk/transsexual.org/home/index.htm
The Royal College of Psychiatrists' Good Practice Guidelines on gender identity disorder. All the official stuff, and well worth reading. http://www.google.co.uk/search?hl=en&source=hp&q=royal+college+of+psychiatrists+good+practice+guidelines+gender+identity+disorder&meta=&aq=2&oq
The Department of Health's resources on gender and equality: http://www.dh.gov.uk/en/Managingyourorganisation/Equalityandhumanrights/Genderidentity/index.htm
Non-binary identities:
Introduction to 'genderqueer' - http://www.genderqueerrevolution.org/gqr/home/about.html
GenderQueer Revolution also has an active Yahoo discussion group - http://groups.yahoo.com/group/genderqueerrevolution/
Message boards and discussions - http://www.whatisgender.net/phpBB3/viewforum.php?f=147
Internet-based support groups:
http://community.livejournal.com/transgender/profile - general international high-traffic LJ community, highle recommended
http://community.livejournal.com/tguk/profile - general trans LJ community, very helpful for UK-specific information
http://www.depend.org.uk/support.html - for relatives, especially good for partners
http://transactiveonline.org/ - for young transgender people and their relatives
http://theangels.co.uk/about/ internet support group for trans women
http://groups.yahoo.com/group/FTM-UK/ - UK FtM Yahoo group (requires a Yahoo account to view)
http://www.ftmi.org/ - FtM International, has links to several groups under the 'Community' heading
http://www.transyouth.org/ - Trans Youth Network, part of Queer Youth Network, the UK's national LBGT youth organisation
Blogs and activism:
http://genderbitch.wordpress.com/
http://questioningtransphobia.wordpress.com/about/
http://birdofparadox.wordpress.com/
Copyright L Demtchenko, 2010. If reproducing elsewhere, please include credit and a link back.
Gender Identity Disorder (previously known as 'transsexuality')
"I think I'm trans: what now?"
"My partner/family member is transgender: what now?"
Transphobia and being a trans ally
How does the "T" fit within LGBT?
Language and inclusivity - for societies, organisations and individuals
Trans meets in Cambridge and London