Library of Congress and Flickr – what a beautiful match
This is so exciting! I use Flickr to house my own photos and I'm thrilled to say we use Flickr at work, too, but what the Library of Congress is proposing is really revolutionary in that a very large, extremely credible and federal (!!) institution is venturing into the world of online community and social tagging and encouraging other libraries and other cultural institutions to do the same by using all that Flickr has to offer.
You can see the collection of over 3000 gorgeous photos (one set from the 1930s and 1940s, the other from the 1910s) here.
It's great for the LOC because they're getting free help with their metadata. It's great for everybody because everyone will have better access to more complete information about a particular photograph and be able to directly contribute. Very exciting! Have I already said it's exciting? 
LOC writes about it here and Flickr writes about it here. David Weinberger (who I found out from) writes about it here.