SIGGRAPH 1991 and 2006 featured demos of the ~3000 member audience playing Pong. Each half of the room controlled their paddle by holding up colored wands to vote up or down. I think the 2006 show also had the audience pilot a flight simulator. It was amazing at how quickly the audience got really good at the games!
Here are some photos. I saw a video somewhere, but I can't find it now.
