The resources covered were
5 minute clinical consult is searchable by systems and alphabetically by disease and by condition.
Diagnosuarus is reachable through AccessMedicine and can be searched by symptom, disease or organ system.
VisualDx is a dermatological, ocular, dental differential diagnosis generator that you can create a DDx by using the differential builder or look up the disease state.
DiagnosisPro is a free online DDx generator that uses key findings to create a differential.