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 mark the anniversary, the Library of Congress is gathering all the most popular photos from everyone involved and showcasing them in four (1
"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.