Critical Mass in Santiago!! I've never participated, but it's a big deal in SF.
Basically one night a month where a huge group of bicyclists take over the streets and promote alternative forms of transportation.
A great way to see more of the city, now I just have to buy myself a bike! My roommate found a place to rent a bike for free for the event, but now I feel as I can't continue living without biking everywhere, it's so much fun!