Santa Claus Conquers the Martians (1964) is considered to be one of the worst movies ever made. It is so bad, that it is actually fun to watch. The filmmakers were probably too embarrassed to renew the copyright, so it's now in the public domain.
The story shows how Santa Claus is kidnapped by Martians to cheer up the Martian children. The rumor is that the movie was shot in abandoned aircraft hangars in New Jersey. It is clear that the movie had an extremely low budget, it even has spelling errors in the opening credits (custume designer).