There's a nice collection of really cool old photos free of known copyright restrictions that has recently been posted on flickr on behalf of the George Eastman House. One can only imagine the wealth of photos in that collection!!! You can find the flickr collection here.
I am so glad that these are "free" for use. Sometimes its tough finding original imagery to use that is legally free of copyright restrictions. I have learned the hard way about these laws, and I am trying really hard to stick by them. It means more painting, more imaginative image searching, and a bit of photography on my own part.