Worth1000.com put together a great photoshopping contest of old WWII propaganda posters.
Along the same lines, only with a political purpose, The Propaganda Remix Project has numerous reworked posters, including some for sale.
Seems like I’ve seen another link like this someplace, but I don’t seem to have it bookmarked.
UPDATE: I found the link; it was Another Poster for Peace. I wrote it before. I linked to the Miniature Gigantic site in that post as well – they have peace posters, too.