Even the Library of Congress is in to Web 2.0 technologies. Partnering with Flickr to mount their photographic collections in a site called "The Commons," the LOC is asking visitors to tag these photos with everyday words describing what they see. LC Subject Headings are great for focusing a search, but they are limited to a controlled language that is sometimes baffling. By allowing visitors to tag photos, this site is using the populist strategy found in collaborative websites called Web 2.0 like del.icio.us, LibraryThing and Flickr itself that aims to create easier ways to find info, in this case visual info.
The photos themselves are labeled as having "no known copyright restrictions" so they can be used for whatever purpose you can devise.
Enjoy the photos, but add some value by tagging the ones you look at so that others can find them more easily.