EveryStockPhoto.com is an image search engine which has catalogued over 1 million free photos from places like Flickr and Stock Xchang. These general purpose image sources are pretty useful, although you really only find the ‘quality’ shots from the more specialised services, so be aware that you tend to get what you pay for. That saying, places like Stockxpert are pretty awesome for design and illustration stuff.
We are now searching over one million free photos! We have been adding creative commons photos as a part of our ongoing efforts, and we will soon be adding large numbers of public domain photos as well. We are busy with a big release improving on.