Public Domain Clip Art

Check yourself, but I believe these links are public domain clip art. Check the Terms & Conditions. More links will be added as found

Christmas Graphics Plus
Not just Christmas. Covers a range of holidays and special events

Hellas Multimedia Graphics
Arranged in themes especially holidays and occasions

JAC Clip Art Archive
A comprehensive themed collection which the site owners believe to be public domain, and originally taken from the University of Kansas Medical Center site - now archived

Public Domain Clip Art Blog
Free isn't always free so read the terms carefully. Includes public domain, royalty free, copyright free, high resolution, images, stock photos and free for commercial use clip art

Wikemedia Open Clip Art

Webweaver
Good selection of free clip art arranged in themes

WP Clip Art
Comprehensive selection of categorised clip art