The Upright Citizens Brigade is an
improvisational theater group, originally from the thriving improv community in
Chicago, now centered in
New York. They are truly amazing.
Most people have only heard of the Comedy Central show they have and are not aware of the fact that the UCB members are actually among the world's most talented improv actors. If you are ever in New York, you can actually see the UCB performing live for free. They have a free weekly show they have been doing for years; more recently they added a second show, where you can call in and reserve a seat for $4, meaning you don't have to wait in a line. Look around on the internet. If you can see this show, go. They simply go up on a small stage with some people they know and do two hours of completely improvised sketch comedy. They have a technique called "the Howard" where they take one single word from the audience, then have one person from the improv group ramble for five or so minutes on personal anecdotes and stream-of-consciousness type stuff related to the word given. The actors then structure the next two hours off loosely what the storyteller said, but warp it so wildly that it's hard to believe the final product wasn't prescripted. Like in the TV show, their sketches in the live show intertwine and interact in weird, crazy ways, and in the end they always somehow all connect together. The trippiness of the final result transcends mere sketch comedy in a way that cannot quite be described.
The UCB also teaches improv classes, again if you happen to be in New York.
They are true masters of and pioneers in guerrilla improv; the only such performance they have done that i have heard of was an incident where they went to random post offices dressed as the unabomber sketch and mailed things, but there were many more i haven't heard of. The strange live-action outdoor sequences that sometimes display during the credits of the UCB TV show are generally hidden-camera tapings of their guerilla improv performances.