Creative Commons Search is a tool that can help you search other databases (Europeana, Flickr, Jamendo, etc.) for images, music, video, and other media protected under a Creative Commons License
Wikimedia Commons contains over 27 million freely usable media files, including images, sounds, and videos. The highest quality items are "Featured" by the site and appear in picture of the day and picture of the year selections.
Europeana is an archive of millions of digitized items from European cultural heritage institutional. Multiple formats (image, video, text, sound) are available
Flickr Commons is a repository for historical images from cultural heritage institutions such as libraries, museums, and archives. Most material has "no known copyright restrictions" and is free for non-commercial use. Make sure you check licenses.
Pixabay is a collection of over 430,000 high-quality public domain stock-like images. All images, except the images watermarked with Shutterstock can be used with no restrictions. Users can "like" or favourite images, providing a means of evaluation.
I-Open is an Open Access Bio-medical Image Search Engine