Sutro Project / Critical Mass
San Francisco's urban traffic take-down brake-down inspired 300+ others around the world. Hard to describe, always lively, and mostly legal - the crew roams free the last Friday of every month at 5:30pm. Costumes encouraged (especially for the extra special Halloween/October ride!) and any human-powered vehicle is good to go.
For the most part, (blocked) drivers of fossil-fueled vehicles are friendly, and shout/horn-toot encouragement, but there are usually a some disgruntled motorists as well. CM's more aggressive participants will deliberately antagonize these road ragers... no obligation to join the escalation :)
Wikipedia: http://en.wikipedia.org/wiki/Critical_Mass
