The major space-related agency in the government, NASA's presence on the Web is voluminous. (In fact, there is so much information on a wide variety of subjects, that regular visitors to the site may want to take advantage of their My NASA feature, which allows customization and bookmarking.)
From their main page you can see recent news items and links to pages customized for different types of users (Students, Educators, Media, etc.) Each of these subsidiary pages contains news items and links to related resources for each audience.
The search engine offered on the main page searches all of NASA's many web sites.
The Galleries tab (at the top of the page) leads to their large collection of videos, images, and podcasts, as well as the schedules for NASA-TV. To connect to the websites of any of NASA's subsidiary labs and centers, look under About, then Locations.
Some additional NASA sub-sites of interest:
NOTE that the Library ceased receiving new tangible government documents in Spring, 2016, and withdrew large numbers of tangible documents during the previous year.
NASA Special Publications. NAS 1.21: 1966- . Some numbers are missing; some were received in microfiche. Many of these monographs contain excellent photographs and report results of space explorations. For historical information, particularly check numbers 4000 and higher, which are in a special NASA history subseries. A particularly interesting set is Exploring the Unknown, #4407 (see below).
Astronauts & Cosmonauts: Biographical & Statistical Data.Y 4.Sci 2:103/I. 1994. Includes photographs; revised occasionally.
Exploring the Unknown. NAS 1.21:4407. 1995. This 6-volume set includes, as its subtitle indicates, "Selected Documents in the History of the U.S. Civilian Space Program." Not all of the pieces included are official or even connected with NASA; there are reprints of magazine articles, fiction and other materials considered influential in shaping American attitudes towards space exploration. A truly remarkable reference, the set includes individual biographical appendices and indexes in each volume. Volumes are available online from the NASA website, in PDF form: Volume 1, Volume 2, Volume 3, Volume 4, Volume 5, Volume 6.
Space Shuttle Challenger Accident: Report of the President's Special Review Board. PR 40.8:SP 1/R 29/V.1-5. 1987. The Board's mandate was to review the accident in detail to establish the probable cause or causes for it, and to develop recommendations for corrective action based upon this review. The first volume contains this review and the recommendations, and the first four of 15 supporting appendices. Volumes two and three contain the remainder of the appendices, and volumes four and five contain transcripts of the hearings held by the Board in February and May, 1986.
The Wind and Beyond: A documentary journey into the history of aerodynamics in America. NAS 1.21:4409. A proposed 6-volume set which examines the importance of aerodynamic development in the evolution of the airplane.
Galaxy graphic from NASA's image gallery.