PubMed has been around for a long time, but it’s such a wonderful resource that it’s worth repeating for those who may not have discovered it yet.
A service of the National Library of Medicine, PubMed includes over 15 million citations for biomedical articles back to the 1950’s. When available, links to full text articles and other resources are provided.
A related resource is MedlinePlus, which offers consumer-oriented health and drug information for patients, family and friends.