I'm gonna second
Hedwig and the Angry Inch, although I don't know if it's old enough to be a "cult classic" yet. But, it does get the same audience participation and shadow actors that you see at RHPS screenings.
I'm also going to have to disagree with a lot of the nominations here. If the movie was released in the past five years, it certainly can't be considered a "classic" yet. (Thus my reservations about Hedwig). And, if the movie was released in mainstream theaters with mostly mainstream actors (Old School, Ferris Bueller's Day Off, Office Space, etc.), it can't be cult.