ART

Art.com - Art reproduction online store with large, searchable image catalog
SILS Art Image Browser - Art, architectural, and museum object images from 4 museums/collections
National Portrait Gallery [US] - shows the most influential characters in United States history portrayed by the finest artists of their generation
 

ASTRONOMY/SPACE

Astronomy Picture of the Day Archive - Pictures along with a brief explanation written by a professional astronomer
Hubble Space Telescope Greatest Hits - Unprecedented pictures of celestial objects
Astronomical Image Library - Helps out by allowing you to search for images of any object you specify.
NASA Image eXchange - NIX returns thumbnail sized images, textual descriptions, image numbers, links to higher resolution images, links to more information, and links to the NASA Center that stores each image
 

Cartoon Images

Search for Looney Tunes Characters.

Animal Printouts - Hundreds of Animals to choose from. They are arranged in alphabetical order as well as by area and subject ( African animals, ocean animals, etc.) 

Nickelodeon Online -You can find Sponge Bob,Fairly Odd Parents, All Grown Up, Jimmy Neutron Boy Genius, Rocket Power, My Life as a Teenage Robot, Rugrats, Chalk Zone, Hey Arnold, and the Wild Thornberrys.

Warner Brothers Official Site - Bugs Bunny, Daffy Duck,  Speedy Gonzalez,  Taz, Tweety and more.

ANIMALS

FWS National Image Archive - The collection includes images of a variety of species including birds, amphibians, reptiles, fish, mammals and plants
Animals at the Smithsonian's National Zoo - The photos in this library are from the vault of the National Zoo
eNature.com Guides to North America - Online field guides covering over 4800 species
Garst Photographic Collection - Thousands of color photographs of animals from around the world
 

HISTORY/SOCIETY

American Memory Collections: Photos & Prints - Distinctive, historical Americana holdings at the Library of Congress
Amazing Picture Machine - Many collections with emphasis on American history and science
Smithsonian Photo Data Base - Topics ranging from Art, to Air and Space, Science, Nature, Technology, History, and People-Places
National Anthropological Archives - Society, government, field notes, maps and much more
Colorado Digitization Program - Thousands of historic images of plants, animals, people and more from Western history.