If you are like me, you cannot get enough free clip art to download and hoard on your hard drive for future creative uses. Take advantage of the impressive array available on Martha Stewart's craft pages.
There's also a great source of vintage clip art available from Dover Publishing, a great tip from How About Orange. If you sign up, they'll send you an email every Wednesday with links to free art.
If you are interested in vintage (ok, some schmaltzy and torrid, use discretion) paperback book cover design, there's more than any little heart could desire available for download at the BookScans Database. I looked but couldn't see any relevant copyright information. Use commercially at your own peril.
Another source for copyright free images is the Library of Congress Prints and Photographs Reading Room. Note: not all of these images are public domain.