Good Show Sir offers an extensive collection of "only the worst Sci-fi/Fantasy book covers". Some are absolutely hilarious, many are disturbing and almost always leave you with a "what were they thinking" look on your face.
The site is heavily commented and the comments are often more entertaining than the cover, itself. The covers can be rated and each one is tagged. Covers are collected from all over the world and reader submissions are encouraged. Worth a look! Below are a few less offensive offerings. Click to enlarge.
Found at Coudal Partners