Emory Douglas Hard-hitting political posters by the Revolutionary Artist of the Black Panther Party
Flyer Design Goodness Contemporary and vintage posters, usually collected by designer, from around the world. [Not currently active, but there’s a great archive]
Occuprint 1000s of posters of the contemporary Occupy movement from around the world. Wonderfully archived and sourced.
The Harry Ransom Center at UT-Austin 10,000+ classic movie posters
USSR Posters Soviet and Russian propaganda and advertising posters, 1917-91
WPA Posters 1930s-40s New Deal-era public works posters, brilliant and graphic
Wrong Side of the Art Trash, exploitation, Blaxploitation, and other cool, bad movie posters
Adult Film Posters Groovy collection of X-rated movie posters, 1960s-70s, via the CineMasterpieces site.