Google Image Search: http://images.google.com/advanced_image_search?hl=en

Wickimedia Commons: http://commons.wikimedia.org/wiki/Main_Page

Flickr: http://www.flickr.com/search/advanced/

BigFoto: http://www.bigfoto.com/

Free Digital Photos: http://www.freedigitalphotos.net/

Free Photos Bank: http://www.freephotosbank.com/

Free Pixels: http://www.freepixels.com/

Freerange Stock: http://freerangestock.com/index.php

Image After: http://www.imageafter.com/