All works created by the US government are not subject to copyright under US law. Be careful with the use of logos, seals and currency. A page with more details about how to utilize government works can be seen here: http://www.publicdomainsherpa.com/us-government-works.html
Google .GOV Search: A robust Google image search restricted to .GOV domains. Verify that the image was created by an employee of the US Government before use.
TotallyFreeImages.com: A collection of over 500,000 US Government images spanning a variety of topics. This site has a simple interface but limited search capabilities.
National Park Service Multimedia Collection: 26,000 photos, 2300 videos, 510 sound and 160 webcam clips from the National Park Service.
National Park Service Historical Photo Collection: A selection of images form the National Park Service archive.
US Fish and Wildlife Service: Photo, video, audio, maps and document archive from the USFWS.
US Army Corps of Engineers: Their digital library contains maps, photos, documents and video.
USDA Photo Gallery: High quality photos of US Agriculture organized by state.
US Antarctic Program: Thousands of photos of the Antarctic that are free for use.
US Department of Defense: High quality military photos.
US Navy: About 200,000 US Navy Photos.
NOAA Photo Library: National Oceanic and Atmospheric Administration photo library spanning centuries covering topics of weather, geography and space.
US Capitol Flickr Page: Links to the US capitol videos and Flickr page. The collection contains over 2000 images that show the promotions, the building, its operations, as well as construction projects.
Whitehouse: Photo and video galleries from the Whitehouse.
National Archives: Historical document archive including the Declaration of Independence, The Constitution and Bill of Rights. It also includes landmark acts, speeches and patents.
National Human Genome Research Institute: Searchable database containing high quality photographs, graphics and video files related to genomic research.
National Institutes of Health: NIH photo galleries and a portal to other health related government media galleries.
US National Library of Medicine: About 200,000 historical medical images.
Bureau of Land Management Flickr: Flickr page of images from the BLM.
United States Geological Survey Multimedia Gallery: Aerial photographs, satellite images, maps and publications form the USGS.
US Geological Survey: Galleries of USGS graphics and volcanos.
NASA Landsat Image Gallery: Satellite imagery from 1972 to the present in clouding images from all seven Landsat satellites.
Energy.gov: Portal to galleries from the Department of Energy’s laboratories.
Environmental Protection Agency Flickr Stream: EPA photo stream
ForestryImages.org: A collection of forestry images from various sources that have been released for educational use.
USDA Special Collections: Historical agricultural documents and images from the 1500s to the present.
NASA Historic image database: High quality images from NASA with historic significance.
NASA multimedia gallery: Main NASA media gallery with Astronomy and space exploration related photos.
NIST Image Gallery: National Institute of Standards and Technology image database
National Science Foundation; A collection of science related images and video.
US Mint: High quality images of US coins and medals.
Department of Defense:
State Department: Portal to various State Department photo galleries including their Flickr Photo Stream.
US Customs and Border Protection:
Federal Bureau of Investigation (FBI):
Federal Emergency Management Agency (FEMA):
Department of Homeland Security: