Copyright free
Wikimedia Commons — http://commons.wikimedia.org/wiki/Main_Page
Library of Congress — (many but not all are copyright free) –
http://www.loc.gov/pictures/
USGS — http://gallery.usgs.gov/
USDA — http://www.ars.usda.gov/is/graphics/photos/
US F&W — http://digitalmedia.fws.gov/cdm/
US Government — http://www.usa.gov/Topics/Graphics.shtml
NOAA — https://marinelife.noaa.gov/media_lib/index.aspx
NASA — http://www.nasa.gov/multimedia/imagegallery/index.html
CDC — http://phil.cdc.gov/phil/home.asp
Special sources:
World Trade Center — http://www.nytimes.com/slideshow/2010/02/10/nyregion/20100210-WORLDTRADE_index.html
Permission probably needed:
Compfight — http://compfight.com/
Flickr — http://www.flickr.com/search/advanced/