I mean those films that are so terrible they are unbelievable, and shock you into (what the...?) type laughter.

Probably one of the most well recognised examples of such cult classic low budget celebrated trash, would be Russ Meyers' Faster Pussycat, Kill! Kill!

There are some really funny old Sci-Fi movies out there too, but some of best/worst the stuff I've seen I can't even remember the names of.

I recall a scene in one fifties Sci-Fi film, where the astronauts had just landed on the alien planet, and decided to relax for a while with a nice fag before venturing abroad! Brill.