I was just browsing Improv Everywhere’s web site, and thought some of you might find it amusing—they’re a group in New York City dedicated to semi-spontaneous public whimsy:
Improv Everywhere causes scenes of chaos and joy in public places. Created in August of 2001 by Charlie Todd, Improv Everywhere has excecuted over 40 missions involving over 100 undercover agents. The group is based in New York City.
I especially liked their Moebius mission. You can find the complete list here, and more information about the group in their FAQ.