Websites:
Historic Photos from LIFE: http://images.google.com/hosted/life
Free Photo Collections: http://www.imageafter.com/
Pics 4 Learning: http://pics4learning.com/
International World Images: http://worldimages.sjsu.edu/
Smithsonian Images: http://smithsonianimages.si.edu/siphoto/siphoto.portal?_nfpb=true&_pageLabel=home
Images from Boston University Libraries: http://www.bu.edu/library/instruction/findimages/index.html
Images from University of Washington Libraries: http://www.lib.washington.edu/types/images/