Judge A Book... by its Cover: cool collection of bad covers images. Here you will find hard-boiled detectives, scantily-clad floozies, unlikely space-ships, grotesque aliens, bizarre religious images, beatnik shockers, hell's angels, skinheads, barbarian heroes, and horrifying monsters... among others.

The Pulp Gallery is an amazing collection of cover art of pulp and pin-up magazines with hundreds of images. Primarily the Gallery focuses on pulp cover art from the 1920's through the 1950's and includes some magazines from beyond those date ranges. (both links via Arboles muertos y mucha tinta)