Online Exhibits - The National Archives
(includes an exhibition called Picturing the Century)
http://www.archives.gov/exhibits/index.html

LIFE photo archive hosted by Google
photos (many unpublished) from the 1750s - today.  sorted by decade.  can also search using the search box
http://images.google.com/hosted/life

Aletheia Photo Collective
photojournalists covering social and humanitarian stories
http://www.aletheiaphotos.com