PublicDomainResources
This will be a list not only of public domain resources, but also "free" (GNU licensed) and open content resources. Feel free to rename this page and the link(s) to it, if you like...I'm being lazy.
Let's go get stuff from these pages and post it (with appropriate source citation, if necessary) on the Wikipedia!
Images of American Political History
US Census Bureau -- has an enormous range of facts and figures which we will find helpful in fleshing out articles about cities and states.