Pics for Learning: Nice for the younger grades. Best for science or social studies topics.

Flickr Creative Commons - Blue Mountains: Easy to search, easy to size, easy to find the attribution.

The Morgue File: Strange name, but great resource.

FlickrStorm: Click 'Advanced' then choose "Photos you can use commercially" to easily find and organize images for many projects.

The World Images Kiosk:  75,000 Images! Most are free to use, but some will require attribution.

Photos 8: Public Domain Photos

Edupic: A website of images created by a teacher for use by teachers and students.

4FreePhotos: This site that offers quite a few artistic high quality images. Many of these would be great for backgrounds

Library of Congress Prints and Photographs: Over 1.2 millions images in this database, great for history projects.  Each image has different licensing, so look closely.

NOAA Public Image Library: A wonderful resource for science themed drawings, maps, graphics, diagrams, and photos.

PD Photo: Great image site for animals, geography, space and plants.