Prodigy
On January 12th, 900 enthusiasts in NYC dropped their pants for the 7th Annual No Pants Subway Ride, with NYPD escorts and crowds so large that the under-pantied participants were spread out over three subway lines, and stretched six per car. Improv Everywhere helped organize similar No Pants events in Boston, Washington DC, Portland, OR, San Francisco, Chicago, Toronto, Adelaide, Australia, Baltimore, MD, Salt Lake City

Here are a couple other campaigns from Imrov Everywhere:

Implications - I'm sure you're wondering what the point of this goofy and (perhaps a little) childish activity is, well, ImprovEverywhere has developed this international event with an eye towards putting a smile on people's faces in an awkward manner. There are rules (no laughing, lewd behavior, or G-strings), but thankfully, many of them do not to be enforced because those participating have genuine respect for the audience.