Their most recent mission was No Pants 2K7, a version of which they apparently do every year. It basically has a bunch of people ("agents") riding the subway with no pants, pretending not to know each other, like they all just happened to forget their pants that day, purely by coincidence. This year's mission was livened up by a panicky cop [eyeroll] although everything turned out fine and they're planning to do it again next year.
Back in 2003 they did a mission called The Moebius which was incredibly cool. They staged a timewarp at a Starbuck's, with several individuals and small groups of agents (again, making like they didn't know each other unless they were directly interacting) performing the same actions over and over, in a timed sequence of loops. Once people started noticing what was going on, it got sort of trippy. :D
Their main web site is here; click on the "Missions" button to get a complete list.