Mirror World — Photographs Of Unknown Origin. This site is packed with interesting found photos, many of them old. I recognize some of them (e.g., von Richtofen) as published works, but many appear to be postcards or obscure snapshots.
The site reminds me a bit of the snapshot exhibition at the National Gallery, except that Mirror World is less edited, more diverse, more profane and full of interesting surprises. Worth a look.
(Lex found it at Coming Anarchy)