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

Wikimedia image search: dig down into subcategories when you search.

Google Advanced Image Search - under "usage rights", select "free to use share or modify".

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

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

Public Domain Images: smaller, less user-friendly site.

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

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

Wikipedia list of public domain resources: the granddaddy of them all - a huge list of links to sites.

FREE VECTORS also has photos, brushes.

Vecteezy: vector images for use in Illustrator.

bittbox: nice gallery of assorted textures, backgrounds.