The Astrophysics Data System (ADS) is a NASA-funded project which maintains four bibliographic databases containing more than 3.6 million records: Astronomy and Astrophysics, Instrumentation, Physics and Geophysics, and preprints in Astronomy.
Nature Publishing Group (NPG) is a publisher of high impact scientific and medical information in print and online. NPG publishes journals, online databases, and services across the life, physical, chemical and applied sciences and clinical medicine with more than 89 journals.
NGC is an initiative of the Agency for Healthcare Research and Quality (AHRQ) , U.S. Department of Health and Human Services. NGC was originally created by AHRQ in partnership with the American Medical Association and the American Association of Health Plans (now America's Health Insurance Plans [AHIP]).
The NGC mission is to provide physicians and other health professionals, health care providers, health plans, integrated delivery systems, purchasers, and others an accessible mechanism for obtaining objective, detailed information on clinical practice guidelines and to further their dissemination, implementation, and use.
The National Academies Press (NAP) was created by the National Academies to publish the reports issued by the National Academy of Sciences, the National Academy of Engineering, the Institute of Medicine, and the National Research Council, all operating under a charter granted by the Congress of the United States. NAP offers more than 4,000 titles in PDF format. All of these PDFs can be downloaded for free by the chapter or the entire book.
The National Library of Medicine (NLM) provides a variety of resources related to the biomedical and health sciences, including databases, bibliographic citations, full text (when available), archival collections, and images.