Most movies in IMDB are accompanied by an image of the movie poster. With the advance search you can select a genre and browse by that genre. Take note that sometimes the image provided is not the original movie poster. For example, if you look at Night of the Hunter the image shown is the cover from the Criterion Collection DVD release. However, if you click on the image you are able to browse through other images and often the original movie posters are included therein.
A movie poster gallery in New York City that collects and sells original movie posters. You can browse/search by director, actor, and artist as well as genre. They have some unique genre categories like "Bikers and Hot rods".
The Academy of Motion Picture Arts and Sciences (the Oscars) has put together an online database of over 6,500 items, including movie posters. You can do a keyword search and limit the results to "posters."