From the Flickr Blog:
…we’ve heard from our community that we’re missing two important designations: Public Domain and Creative Commons 0 (CC0). Many members of our community want to be able to upload images that are no longer protected by copyright and correctly tag them as being in the Public Domain, or they want to release their copyright entirely under CC0.
So, starting today we’re happy to support these two new options. One of the first accounts on Flickr to change its designation was SpaceX, which has uploaded more than a hundred gorgeous images of its launches. These extraordinary photos are now available for others to freely use, enhance, and promulgate without restriction under copyright law.
As with any license you choose, it’s important to be informed about what the Public Domain and CC0 designations mean. You can find more information about what it means when a work is part of the Public Domain here (note: this designation is not applicable everywhere), and you can read up on the CC0 designation on the Creative Commons info page.
Read the Complete Announcement