The Library of Congress is the US's oldest federal cultural institution and serves as the research arm of Congress. It is also the largest library in the world, with more than 130 million items on approximately 530 miles of bookshelves. The collections include more than 29 million books and other printed materials, 2.7 million recordings, 12 million photos, 4.8 million maps, and 58 million manuscripts. You can search the library by author, title, ISBN and keyword, and access digital images depicting people, historical events and architecture from around the world. There is also a guided search option and a good explanation about searching with Boolean operators.
The second US Government website to be nominated is NASA's Earth Observatory. This site provides satellite imagery and scientific information focusing on the Earth's climate and environmental change, under the headings atmosphere, oceans, land, life on Earth, heat and energy, remote sensing and pioneers in geoscience. Materials are freely available for use and publication, except where copyright is indicated. There is also a glossary explaining some of the terminology used on the site.