PhotoPin: Creative Commons Photos For All Your Stock Photo Needs
I've been looking for something like this for years: a central, easy-to-use site for stock photos. Called PhotoPin, the site features a search engine that trolls Flickr for pictures. Some of them are also available to buy, but the vast majority are CC licensed. Obviously the site (it's more a widget, really) reminds you to link back to the original image, but if you've ever tried to find an acceptable image for "ninja stormtrooper" then you'll understand the value of the site.