Where to find free graphics

I am looking for good sources of clip art and graphics that are available for use. I know that Rini Templeton's site was suggested:

http://www.riniart.org

does anyone know of other good sources of free graphics? I have found other good art at:

http://www.favianna.com/ (under stickers ... she explains how she wants to be credited if you use her graphics)

http://www.politicalgraphics.org/links.html

But I don't know of other good places to find clip art.

thanks,
amanda