It was only 2008 when Flickr Commons opened it's digital doors in collaboration with the US Library of Congress. The idea was that it would be a useful educational, allowing better access to images withheld in public collections. Initially the Commons featured a mere 1,500 public domain images and since then they've acquired over 250,000 images from 56 different libraries, archives, and museums. To celebrate 5 years of public domain photographs, Flickr has been compiling sets of some of the most favorited images which you can check out at the following galleries: 1, 2, 3, 4
The Fifties in 3D - The National Archives UK
Sponge diver John Gonatos: Tarpon Springs, Florida - State Library and Archives of Florida
Dog with a pipe in its mouth - Llyfrgell Genedlaethol Cymru / The National Library of Wales
The Eagle Nebula (M16): Peering Into the Pillars of Creation (A nearby star-forming region about 7,000 light years from Earth.) - Smithsonian Institution
Eénwielige motorfiets / One wheel motor cycle - Nationaal Archief