Tumblr site with fabulous high-res images. Sign up for their e-mail alerts and get ten new photos e-mailed to you every ten days. These photos are all in the Public Domain, which means they are not under copyright so you can use them however you would like!
Google Images has millions of stunning historical photos from LIFE Magazine from the 1860s to the 1970s. You can use these images for personal, non-commerical purposes.
The British Library has uploaded over 1 million public domain images into Flickr for your use!
Search for thousands of images that users have uploaded to Flickr and made available for reuse.
Limit your Google Images search to Creative Commons-licensed photos.
Searchable archive of usable photos from DVIDS, a public relations arm of the U.S. Army.
Images related to modern languages and cultures, all vetted by faculty at Oberlin College.