It's amazing how useful the old adage "Keep it simple stupid" is. One of the most entertaining interactive movie experiences I've ever encountered has to be the (very offline) Sing-along Sound of Music, the karaoke print of the film/variety show regularily held at the Prince Charles Cinema in London.
Singalonga.com must be doing something right. They've just released their fourth film - Singalonga Annie.
"Expect all the Singalong regulars – free goody bag, magic moments, fancy dress competition and the chance to cheer, boo and hiss to your heart’s content. And, if you can’t get your costume together, remember, it’s what you wear from ear to ear and not from head to toe that maaaatters..."