New favorite site
I highly recommend checking out Shorpy:The 100-Year Old Photo Blog.
Along with posting some of the best historic photos such as . female factory workers circa 1900
or a beautiful photo of the Chicago and North train yards in Chicago during the 1940s.
or Johnny Weissmuller in an Illinois Athletic Club swimsuit circa 1922
There are also these little gems, such as finding the hidden contents of a letter on a Leave it to Beaver episode. It's Priceless... The Leave It To Beaver Letter.
(Thanks, Towerload, for the tip)