http://www.clker.com/ – Creative Commons Public License!
http://thestocks.im/ -> a collection of links to a LOT of stock photo sources.
http://photopin.com/ – flickr search
http://compfight.com/ – flickr search
http://unsplash.com/ – totally free random hi-res photos
http://www.google.com/advanced_image_search – allows specifying license
http://www.everystockphoto.com/ – search on many places
http://www.stockvault.net/ – restricted on licensing
https://commons.wikimedia.org/wiki/Main_Page – mostly from wikipedia
http://www.bad-neighborhood.com/creative-commons-image-search.htm – Tool to help facilitate searching Google Images for Creative Commons content.
We’ll update the list little by little and will also add reviews for these sites once we get to know them better. If you got another suggestion, do let us know!