Diagnosaurus: From Access Medicine. Search this site by disease or symptom to retrieve possible differential diagnoses.
Rational Clinical Examination: The articles in this series of systematic reviews from JAMA delineate definitive diagnostic strategies for patients presenting with various symptoms.
Duke University Health System Laboratory Test Compendium: Information about procedures at DUHS, normal values, and some interpretive information about test results.
Lab Tests Online: A peer-reviewed site with lots of information about lab tests, why they are done, and what the results may mean.From the American Association for Clinical Chemistry.