Also, check out Improv Everywhere, a "New York based group that causes scenes of chaos and joy in public places". This is their YouTube channel, and this is their website. Oh, and Katie takes photos for them which she sometimes posts on her blog.
On a side note. I wish I was in Florida right now. I'm following the Wizarding World of Harry Potter videos and links on Mugglenet, and they all seem so ah-mazing! I think I need to plan a holiday to Florida with a friend. Hopefully after our degrees are finished [she's doing Commerce]. Btw, thanks for putting up with my Harry Potteryness here. I'm obsessed! :)