Improv Everywhere

Highly, HIGHLY recommend this YouTube channel. They specialize in getting a group of people to simultaneously do something weird in public. The task is usually something that violates a social norm, like hundreds of people suddenly freezing in place at the same time in Grand Central Station. Reactions of passers-by are taped. The "missions" bring amusement and laughter. Also, it's based on interesting groupthink psychology and explores the "Are you in on the joke?" dynamic. But mostly, it's hilarious and fun!

http://www.youtube.com/user/ImprovEverywhere