Improv Everywhere, as described on their own website, "causes scenes of chaos and joy in public places. Created in August of 2001 by Charlie Todd, Improv Everywhere has executed over 70 missions involving thousands of undercover agents." But while they seem quite similar to a well organized flash mob, they differ in certain key details, namely the amount of time an action is performed as well as the anonymity of those involved.
Below is, if I may say, one of Improv Everywhere's finest achievements: The Food Court Musical.
The Food Court Musical