CharityChoice Comprehensive directory of UK charities. Includes many medical and health related organisations. A directory at a very reasonable price. Kids Link A UK site dedicated to providing information and support for parents of Children with 'Special Needs'. Many of the links are to non-UK sites and are not confined to just children's problems.
Many links are for conditions which are rare.
CancerBacup is an extensive provider of cancer information in the United Kingdom. You can search their database of over 600 UK wide support groups and cancer patient organisations to find help specific to your needs and where you live. It is intended to help you find and use these groups. Extensive listing of US web sites. Not just genetic - some disability sites listed too.