Improv Everywhere

Improv Everywhere is a group that started in New York (and has now started in many other countries as well) where hundreds of people come together for elaborate improvisational acts in public places. Some of my favorites are the ones where 177 people all freeze at the exact same moment in Grand Central Station for 5 minutes,"Food Court Musical (where several food court employees hijack the sound system and perform a misical number) or the one embedded here. They have a group in San Francisco, but as far as I could tell most of the SF pranks just involve dressing up in costumes and blocking points of egress.