http://www.morguefile.com/ This site has tons of reference photos that are free and open to the public. It's purpose is to provide free high resolution reference material to instructors, artists, etc.

It also contains lesson on various photography aspects such as aperture and speed, lenses, etc. Would be great for a photography class, or even a web design class.

http://www.burningwell.org/
This site allows you to search and use photos for any purpose that have been uploaded by photographers. You do not have to pay to use them, or even acknowledge or give credit to the BurningWell repository or original photographer.

This would be great to allow students to find photos for projects or designs. Most of the pictures are grouped in albums by categories, so some searching for the right photo would be involved.