If you've never heard of Improv Everywhere, you'll be pleasantly surprised and amused after a visit to their extensive site full of links to ingenious & creative social projects
(some would came them 'pranks'). These folks incorporate modern technology, social networking, and group dynamics to engineer some very unique and entertaining 'events' mostly in urban areas around the U.S. All of the content is better to see than to read about, clearly, so it's a must to check out their site at http://improveverywhere.com/ . Enjoy the evidence that in the masses, there is plenty of room for fun and infinite power for change.
Some of my favorite participatory works are:
Slo-Mo Home Depot
Sychronized Swimming in Washington Square Park
No Pants 2K6
Check these out; you'll surely smile!