You've probably already seen this, but I still wanted to mention it, as I think it's kind of interesting. The Library of Congress has put thousands of their images on Flickr, asking for help in meta-tagging them with labels. Sort of a cool little project to help build the public information trust.
They explain the process on their blog: http://www.loc.gov/blog/?p=233
The Flickr page is here: http://www.flickr.com/photos/library_of_congress/
