Creative Commons image search: lets you access multiple sites from one page.
search.creativecommons.org/

Wikimedia image search: dig down into subcategories when you search.
commons.wikimedia.org/wiki/Category:Images

Google Advanced Image Search - under "usage rights", select "free to use share or modify".
images.google.com/advanced_image_search

Flickr image search - be sure to check off "Only search within Creative Commons-licensed content".
flickr.com/search/advanced

Stock.xchng: "the world's best free stock image site". Skip past the "premium" paid options at the top.
sxc.hu

Public Domain Images: smaller, less user-friendly site.
public-domain-image.com/

PD Photo: small site; some not public domain, read each license.
pdphoto.org/

fotogrph: only about 200 images, but offers background textures!
fotogrph.com

Wikipedia list of public domain resources: the granddaddy of them all - a huge list of links to sites.
en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources



FREE VECTORS


allfreedownload.com: also has photos, brushes.
all-free-download.com

Vecteezy: vector images for use in Illustrator.
vecteezy.com

bittbox: nice gallery of assorted textures, backgrounds.
bittbox.com/gallery