A decent list up at the Telegraph, of the best of cult fiction.
You can almost predict half of this kind of list. Yep, Catch-22 and Slaughterhouse 5 make it. A couple more genre books, too, including Dune and Hitchhiker's Guide. Surprised there's no mention of Philip K. Dick. As for more mainstream choices, it contains one of my favourites, the Alexandria Quartet, so I'm a happy chap.