Copyright Free Images

http://www.copyright-free-images.com/

This site has royalty-free, public domain images for personal or professional use.  There are many photos organized by category and sub-category.  The site does say that it would be appreciated to please consider linking back and giving credit, although it does not say it's required.

http://www.freedigitalphotos.net/

This site has many, many photos and images free for use in almost any setting.  In the Terms section of the site, it does say that if you're using free images you need to publish the name of the creator adjacent to where the photo is used.  This is not required for purchased images.  This site seems to only allow downloads for Windows users.  Sad news for us Mac folks.

http://www.publicdomainpictures.net/

This site has free, public domain photos for both commercial and personal use.  The site does caution that if you are using images for commercial use, some require a model or property release.  This is the first site I've seen in my search that has the option to upload your own photos.