The folks at Flickr and the Library of Congress have worked together to create The Commons.

Basically they've taken a subset of the images in the Library of Congress which have no known copyright restrictions and posted them as a collection on Flickr.

Where do you come in?

They're asking / allowing users to describe and tag those images, which will enable them to be found and shared with a global audience.

It's a great collaboration and a beautiful way to spend some time, and browsing a few of the photos it looks like they are already getting tagged well.