This news just fills me with smiles; the Library of Congress in collaboration with Flickr has posted thousands of copyright-free photos and asked Flickr users for their input. If you've ever hesitated getting a Flickr account, now is the time.
Flickr hopes that the LOC will be the first of many institutions and in anticipation, has created the Commons to house these images in need of metadata.
If you need me in the next few hours... I'll be browsing and tagging what I can.