A site for open source vector (SVG) clip art: http://www.openclipart.org/
More art, sound, video for games (under various licenses): http://opengameart.org/
The Wikipedia is building a good collection, many are CC and Public Domain: http://commons.wikimedia.org/
A thousand or more 16x16 icons under creative commons license: http://www.famfamfam.com/lab/icons/silk/
tags: clipart, clip art, icons, free