Open Clip Art Library
A brand new version of the Open Clip Art Library just came out!
26175 (and growing) scalable vector graphics, all available under a public domain license: they may be freely reproduced, distributed, transmitted, used, modified, built upon, or otherwise exploited by anyone for any purpose, commercial or non-commercial, and in any way, including by methods that have not yet been invented or conceived.
http://www.openclipart.org