GARD: Genetic and Rare Diseases Information Center
OMIM: Online Mendelian Inheritance in Man: An Online Catalog of Genes and Genetic Disorders
MedlinePlus: Health Topics National Library of Medicine
Covers symptoms, causes, treatment and prevention for over 900 diseases, illnesses, health conditions & wellness issues.
MedlinePlus health topics are regularly reviewed and updated.
Merck Manual of Diagnosis and Therapy for medical professionals