A catalogue of images, manuscripts, etc maintained by the American Antiquarian Society. What an awesome bunch. Will be good for any aspect of American social history, I would imagine.
A series of links to Chinese history collections, provided by the National Library of China. There are a few dead links and the Chinglish can be amusing, but seems as though there are some useful sources in there for Chinese history.
This is a collection of images on flickr of manuscripts, mostly it seems from the seventeenth century. It's maintained my a rare book and manuscript library at Yale.