Search the Creative Commons -- searches across more than 300 million images from open APIs and the Common Crawl dataset. It goes beyond simple search to aggregate results across multiple public repositories into a single catalog and facilitates reuse through features like machine-generated tags and one-click attribution. More info here
Content that has a Creative Commons license is free to download, adapt, distribute, and transmit without having to ask permission. Depending on the license, however, there may be certain conditions: you may only be able to use the content for educational purposes, you may have to give attribution, etc. (Licensing characteristics can be found below). Because licenses vary, always be sure to check the exact terms of the license before using an image.
Attribution: others can use the work however they like, so long as they give credit
No Derivative Work: others can copy, display, or perform your work, but it must be verbatim
Non-Commercial: others can use your work, but for non-commercial purposes only
Share Alike: others can distribute derivative works, but only under the same terms as the original license
Browse the Internet Archive's Image Collection -- This library contains digital images uploaded by Archive users which range from maps to astronomical imagery to photographs of artwork. Many of these images are available for free download.