Pixabay is a site that allows you to search for public domain images.
Open Clipart is a site that allows you to search for clipart that is released to the public domain and can be used free with no restrictions.
Open Photo is a site that allows you to search images that are shared for free under Creative Commons licensing.
MorgueFile is a site that allows you to access free photos. Depending on use of picture, photographer may ask credit line be given with picture.
Pics4Learning is a site that contains copyright-friendly images that students and teachers can use.
Free Tiiu Pix is a site that is put together by a retired teacher. Hundreds of categorized images take by Tiiu personally.
Public Domain Pictures provides a repository of free public domain images.
Wikimedia Commons is a database of freely usable pictures.
Public Domain Photos is a site with 5,000+ photos and 8,000+ clipart that can be used for personal and commercial use.
Flickr allows you to refine your search for public domain images. Similar to Google Images, use the advanced search feature to filter public domain images.
Every Stock Photo is a site that allows you to search for free images. Once you find a picture you like, you can view the pictures license by clicking on the license icon.
FreeDigitalPhotos.net is a site that allows you to download two different resolution of pictures. You can download low resolution pictures for free. To download the high resolution photos you just need to credit the photographer. FreeDigitalPhotos.net even provides guidelines on how to credit correctly.