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