(US Federal Govt.) Centers for Disease Control and Prevention "In general all information presented in these pages and all items available for download are for public use. However, you may encounter some pages that require a login password and id. If this is the case you may assume that information presented and items available for download therein are for your authorized access only and not for redistribution by you unless you are otherwise informed."
(US Federal Govt.) Medline at the NIH copyright terms here indicating materials are in the public domain unless otherwise indicated
(US Federal Govt.) USDA Nutrient Data Laboratory Contains nutritional information on a large number of comestibles. From FAQ: "USDA food composition data is in the public domain and there is no copyright. We would appreciate it if you would list us as the source of the data and when possible we would like to see the product which uses the data or be notified of its use."
(US Federal Govt.) Mental Health: A Report of the Surgeon General (needs positive confirmation before use: some diagrams within the text say "used by permission", which implies that they are copyrighted -- however, the rest appears to be written by U.S. govt. employees)
(US Federal Govt.) The NCI thesaurus is a massive public domain repository of definitions of medical terms