Perseus, a major resource for classical subjects, especially texts. See especially the sanctuaries:

Medicina Romana a French site with resources for Ancient Medicine

Bulletin de Médecine Ancienne (Ancient Medicine Newsletter: most of the site in French)

S.A.M.'s list of dissertations in the area of ancient medicine

Diotima, with extensive references to medical matters, especially gynecology, and including the medical sections from the online edition of Lefkowitz and Fant's Life of Women in Greece and Rome

Medhist: a very useful portal for the History of Medicine

Emory University's MedWeb listing for History of Medicine

MedWeb plus: 'A free service to help you find health sciences information quickly and easily' which also has a number of texts and journals.

BioMedLink, which describes itself as 'an evaluated and annotated database of Internet resources for biological and medical researchers.'