Flickr and the Library of Congress have teamed up to host part of the LoC's image collection online.
The result? Gems like this one:
And this one:
You can view many more Library of Congress images on Flickr, and you can read more about the Commons project. The LoC also has a searchable online catalog of photos.
I'm a sucker for old photographs, so I'm in hog heaven right now.