Night of the Comet
This movie is one of those rare ones that is so absolutely bad that it ends up being fun to watch. A comet goes by the Earth and all the people looking at it are either reduced to a pile of powder, or turned into a zombie. Some people survive, including a ditzy blond and her older sister.
So what you have is a science fiction-type movie mixed with a zombie movie. It's certainly not something you run into every day, but it's actually fun to watch.