If you're looking for royalty-free images for websites, videos, or school projects, CompFight.com is a great tool that searches Flickr for photos that have been made available by the content creator via the Creative Commons license structure. This is a much better alternative to a Google Image Search, especially in schools.
Alas Media shared this information at a workshop at BLC10. You can roll over the search results to see the image resolution. When you click on an image you'll be taken to Flickr's website where there is an option to view "all sizes" that are available for download. You will need a Flickr account (free) in order to download the photos.
Header photo by Robert S. Donovan