If you don't know what Flickr is, its a photosharing website, like Photobucket or Picasa. You log in, upload photos, share them or make them private, tag them, group them, post them on your blog or your Facebook page or whatever you want to do with them.
The Library of Congress (the big poppa of libraries, you may have seen it in National Treasure 2: Book of Secrets) has uploaded approximately 171,000 photos to Flickr and asked the public to view, tag and comment on them. The results are fascinating, and you can see it, for free, here: http://www.flickr.com/photos/Library_of_Congress
To view a webcast created by the LOC, visit this link: Opening the Photo Vaults: A Web 2.0 Pilot Project to Enhance Discovery and Gather Input for the Library's Photograph Collections" (presented Jan. 29, 2008).