The Flickr Commons is one of the things that's unique to Flickr, a place for public domain photographs on a major commercial website. With the public domain becoming largely anachronistic today, it's a miracle that things like this still happen.
The Commons started exactly five years ago with 1,500 photos from the US Library of Congress, the first partner. A lot more organizations have joined since bringing the number of photos in the Flickr Commons to 250,000 photos from 56 partners from around the world.
"To celebrate in a way that recognizes the wonderful diversity and quality of Flickr Commons photos, we asked Commons member institutions to send us links to a few of their most viewed, commented, or favorited images to create this gallery," the Library of Congress explained.