Since somebody mentioned Rocky Horror, I have to mention
Shock Treatment - not truly a sequel to Rocky, but a follow up of sorts. Brad and Janet Majors reappeared, though they were played by different actors.
I've seen a horrible version on TV with all the songs edited out (WTF?) I guess there was a music rights issue. You used to be able to rent
Shock Treatment on VHS - not sure if it made the transition to DVD or not.
http://www.youtube.com/watch?v=z-xRAvVWp5w
Since the TV version of this thread mentioned
Police Squad, we should probably cite the movie
Airplane! from the same producers (Abrahams, Zucker, and Zucker) as the 'grandfather' of this genre. In fact, Leslie Nielson's most famous quote comes from this film -
"I am serious, and don't call me Shirley."
I never understood the criticism of
Police Squad - that you had to "actually watch it." To me, the ridiculous jokes and sight gags came so fast and furious that even if you missed half of them, it was STILL funny. It's not like the plots were complicated...