Originally Posted by ATM
Because most of the responses have been fairly recent movies, I'll pick an old one: Duck Soup.
I'm with you on that one (and also whoever mentioned Murder By Death - the interactions between Alec Guiness as the blind butler and the actress whose name I've forgotten as the deaf/dumb cook are hilarious).
I agree with a lot of the obvious and frequently repeated choices: Airplane, Caddyshack, Planes, Trains, & Automobiles, early Woody Allen, etc. Possibly omitted: It's a Mad, Mad, Mad, World; Strange Brew;
Also, while the movie overall was just ok, the "Freebird" scene in Elizabethtown made me laugh/cough up a lung.