Even if you don't know what Improv Everywhere is, I'm sure you've seen some of their videos online.
Improv Everywhere is a comedic performance art group based in New York City. The group carries out pranks, which they call "missions" or "happenings" in public places. The result is usually shocking and hilarious.
Example:
I was reminded of Improv Everywhere this morning while listening to NPR (http://improveverywhere.com/2010/07/14/star-wars-subway-car/). An old friend of mine was involved with the group a few years ago. I haven't talked to him in a long time, so I'm not sure if he still is. I like to think so when I see things like this ...
I like those videos. I am gonna check them all out now :D