Always one to be behind the times, I'd not heard of Improv Everywhere until today, but a quick scan of the website explained an event I'd unwittingly witnessed a few weeks ago: No Pants 2k8, where hundreds of people in seven cities around the world took off their pants (that is, trousers -- in London I once made what I thought was an innocuous comment about "pants" and everybody thought I was making a ribald comment about underwear) and rode the subways. I'd ridden a train with one of these groups, and assumed they were participating in some sort of marathon. Or something. I don't know. You see weird stuff in NY all the time.
But the latest Improv Everywhere event is marvelous -- be sure to watch the video of Frozen Grand Central. Hilarious and beautiful. Long may they improvise!