http://www.nlm.nih.gov/medlineplus/
Free information on a wide variety of medical conditions from the American National Institutes of Health with a Medical Encyclopedia. Searchable and no registration required.

http://www.nhs.uk/
The web site of the UK’s National Health Service, which has its own on-line health encyclopedia. It also has information on other health-related topics.

http://www.merck.com/pubs/
The web site for the Merck manuals – a free on-line web resource covering a very wide range of medical topics. Some are for the use of the general public while others are intended specifically for health care professionals.

http://www.medicinenet.com/script/main/hp.asp
A searchable web site covering a range of medical topics, primarily for the lay person.