I like old science fiction art as much as I like old science fiction.
There are plenty of sites that show off vintage covers, too. Galleries range from visual indices of cover art to images from famous pulp magazines. For the true fanboy, you can purchase CD-ROMs full of the covers. Other sites target a more specific audience. Say, that of a specific artists or a specific author (this last one, for Philip K. Dick, has to be the most comprehensive collection I’ve ever seen).