Flickr has a section, called The Commons, where museums and archive can post their photos to share. Institutions such as The Library of Congress, The State Library of South Wales, The National Galleries of Scotland, Imperial War Museum, The National Medium Museum, George Eastman House, and Nationaal Archief have posted part of their collections online. In general, most of these photos have no copyright, due to expiration.
These groups are great places to find photos because, just like the rest of the photos on flickr, you can search the tags, titles or descriptions to find photos. When you find a photo, if you have a flickr/yahoo account you can comment on the photo - many people comment with wikipedia links to describe the person, place or thing in the photo.
But searchers shouldn't limit themselves to the commons when searching for family and genealogy photos. Small museums and archive might not be connected with The Commons, but may still have a flickr account. I found out that a local archive, the Kenan Research Center, has a flickr account. Searching for "snow" and "atlanta" turned up a variety of photos that they have posted online. I'm always looking for photos of atlanta to try and find photos of where my ancestors lived. I was very happy to find these.
PS: If you have an unusual or not-so-popular surname make sure to search for it. I just found senior portraits of my aunt and uncle from a fellow classmate who's scanned his yearbook into flickr!