Creative Commons Stock Photos

Compfight is a free online tool which makes the massively overwhelming Flickr pool of images into a source of possible stock photography (or as they say for "comps, inspiration, or research"). By allowing you to search for images by tags or descriptions, that are Creative Commons licensed, you can easily find photos that may be available for use free of charge (with proper credit, check the license first). A great option for small/non-profit clients with low budgets.