Genetic disorders

Online Mendelian Inheritance in Man (OMIM) Extensive, information on all aspects of such diseases, with references to original literature.
GeneCards Takes a different approach, providing outline information with links to pages and sites with further details.

Microbiology

All the virology on the WWW
Bugs in the News Background information and links to other sites on microbes, particularly those of current interest. 
The Bad Bug Book Information on food-borne pathogens

General

MedLineplus Range of information and links for many medical conditions
PubMed The MedLine search facility, allowing search of abstracts of all medline reviewed journals
PubMed Central Free access full text archives of a range of medical and related journals
Highwire Press A more extensive full text archive of scientific journals
ISTH Sub Committee minutes 

Organisations

BIVDA British In Vitro Diagnostics Association
BNA British Neuroscience Association 
BSHT British Society for Haemostasis and Thrombosis
HPA Health Protection Agency
IBMS The Institute of Biomedical Science 
NEQAS United Kingdom National External Quality Assessment Service
NHS National Health Service - NHS Gateway