Improv Everywhere
Need a good laugh? If you're anything like me (God help you), you might get a chuckle out of Improv Everywhere, based in New York. Although what they do isn't technically improvisation, it is hilarious. After they've completed a mission, the participants (agents, as they call themselves) write about their exploits at the IE site. They also post pics and video taken at the scene.
One of my favorite missions was the Moebius, in which the agents created a "time loop" for unsuspecting Starbucks customers.
I guess what I like about IE's humor is that it reveals the absurdity of contemporary life by disrupting its monotony, usually in a good-natured way. For me, reading the agents' accounts of each mission is even funnier than watching the videos.
I wish we had Improv Everywhere, or something like it, here in Ann Arbor. I'm not the type to initiate something on that scale, but I'd definitely take part in a mission or two.