Flickr was -- and maybe still is -- very useful for the computer vision research community.
http://yahoolabs.tumblr.com/post/89783581601/one-hundred-mil...
Yahoo really let themselves get leapfrogged on that one.
As for programmatic access -- Flickr has a good API interface.
I don't use it anymore, but SmugMug.
I hardly upload anything to flickr anymore because the interface for that is so slow.