MEDLINE:

This is the US National Library of Medicine's World Wide Web site. This extraordinary database accesses over 9 million citations in MEDLINE (with links to participating on-line journals) and other related medical databases. Every significant program of the library is represented, from medical history to biotechnology. This is the world's largest biomedical library.  

This baby's got it all, in English and Spanish (but not French). You can search for disease conditions, read FAQs, go to journal articles and abstracts, and follow links to other sites. A very comprehensive and techno site which is recommended mainly for health care professionals. But, if you have a Medical Dictionary handy it will provide you challenging and informative medical reference reading.