The International Vintage Poster Dealer Association put together a show of "Propoganda, Protest & Political Posters" in Fall 2006 that I think is pretty cool. Check more posters out here.
I really love this kind of archive because it represents how passionate people can be. When i was at the Student Union Gallery at UMass, I curated a show with political poster art from a collection in Boston. The posters were part of the show -the other half were prints made by Stan Sherer from archived negatives of UMass students from the 1960's era. It was a powerful show. Lots of similarities between the campus then and in 2006.