Air/Military

FAA Airport Photography
Dryden Aircraft Photos
Military Planes
Dept. of Defense
US Air Force
US Navy Fleet
US Navy Air
US Army Freestock, patriotic, nature, sports, history, science

Arctic General

NSIDC National Snow and Ice Center
NOAA
NOAA Arctic Gallery
NOAA Bering
Indigenous People

Arctic, National Park

Reindeer
Caribou
Caribou 2
Arctic National Wildlife Refuge

Arctic Wildlife 

Caribou
Caribou 2
Caribou 3
Caribou Migration
Wildlife Refuge
Animals, humorous
Library and Archives of Canada
Promotes Oil Drilling

Ecology

Ecology

General/History

Copyright Free Travel Images
Stock.Xchng general collection
Freeimages UK, variety from technology, home, buildings, travel, objects,sports,nature,food,workplace,space,seasonal,medical
Freestock, Animals,Christian,Egypt,Israel,Near East, Rome, scenery, sky, weather, wildflowers,plants
ibiblio, Public library, art, recreation, geography, history, language, literature, sciences, philosophy, technology
Library of Congress, architecture, history, people
Historic Photos and Photo Collections
Historical,Edison,appliances,astronomy,autos,aviation,battery,cement,duplicating machines,electric light&power,energy,etheric force, geographic,medicine,motion picture,Naval,ore milling,patents,people,phonograph,radio,rubber,telephone,typewriter
US Dept Education, search for photos&resources
National Gallery of Art
Low Fee-Public Domain Images from researcher
Word Press Free Stock Images

Health/Body

Center for Disease Control, public health image library PHIL
American Medical Association
Center for Disease Control, health, genetics, safety, disabilities
National Library Medicine, human body
Visible Human, Inner organs
Parasite of Public Health
US Dept Health & Human Services

Ocean and Atmosphere

NOAA photo library index
NOAA marine sanctuaries
NOAA weather

Space (Cosmos)

Astronomy Picture of the Day, photos of space
Astronomy picture of the day (Slashdot)
Community Collection, NASA NOAA (ocean, atmosphere) FWS (fish, wildlife)
European Space Agency
National Optical Astronomy Observatory, stars, nebulae, solar, planets
National Air & Space Museum, solar system, spacecraft, scientists
NASA astronomy photo search - (Public Domain)
NASA, Hubblesite Gallery - (Public Domain)
NASA JPL - (Public Domain) planetary, Mars exploration
NASA JPL Space Gallery - (Public Domain) Images taken by spacecraft as they study Earth and other planets, moons, small bodies like comets, and distant stars and galaxies.
NASA multimedia highlights - (Public Domain)
NSSDC Photo Gallery
Search page NASA multimedia
Solar system, images by Hamilton are copyrighted

Transportation

US Dept. of Transportation

Wildlife/Agriculture

US Fish & Wildlife gallery, regional image libraries
US Fish & Wildlife, mountain, prairie wildlife
US Dept. of the Interior, fish, wildlife, US Geological survey, minerals, national parks, mining, water.
US Army Corps of Engineers
Bureau of Land Management, land, animals, plants & fire
ARS Image Gallery, USDA Agricultural Research Service
Canadian Aboriginal Artefacts
NPS National Park Service
NPS Yellowstone Park
NPS Yosemite
National Prairie Wildlife Research Center
Great Lakes, environment, economy, education, maps, tourism

Media Links

Wikimedia
Creative Commons
Free Pictures Image Search
Cheap Stock:
     istock
     photos.com

Copyright Free Stock Photos

Stock Exchange (highly recommended) 
Morguefile (highly recommended)
Image After
Free Photos Bank
OpenPhoto
BigFoto
USDA
NASA
NOAA
Earth from Space
FWS1
FWS2
Picture Station
Geek Philosopher
Free Images
Pixel Perfect Digital
FreeFoto
Image Base
Cepolina
Amazing Textures
Texture Warehouse
PDPhoto
Image
Blowout
Piotrpix 
Mandragora 
Fotogenica 
Mayang
Free PhotoStation
Cromavista 
Afflict 
Creativity 103 
The Photo Repository 
Aarin Free Photo 
Stockvault  
Designpacks 
Unprofound 
Four Bees 
Insect Images 
Free media Goo 
Burning Well 
A digital Dreamer  
Freestockphotos.com  
Freewebpics  
Lightmatter  
Majestic Imagery  
Triparalbum  
Demny Onink  
Twice Pix  
AboutPixel  
Woophy  
Pixelquelle  
Litetek  
PIX  
tOfz  
Fontplay  
Orangetrash  
ArtFavor 
ImageTemple 
LogoDesignweb 
Fotodatebank 
Bajstock 
Pixelbag 
NWYH Stock Images
PhotoCase
NPS Digital Image Index
Gimp-Savy
DeviantArt

Finally don’t forget to visit
Yotophoto Free photos search engine
and Flickr (be sure to check the licence before using any of the photos).

Smashing Magazine did a contest to accumulate free headers for blogs. Most of them are big enough for UPS websites.  Enjoy!