flashmobs

They exist in various shapes and sizes, but I love every single one... Flash-mobs.
It's probably to do with the fact that it's an "in-crowd" thing: I was there, I was part of it...

wikipedia
http://www.flashmob.com/
http://www.flashmobs.info/

Here's some nice examples:
pillow fight, here's the same fight, but from another angle
red light, green light, yellow card, dusting architecture,