Complex mass protest arranged via website

In protest against the war on Iraq, Hungarian peace activists set up a website that enabled them to plan a complex mass action.

Individual protestors could sign up to the website, pledging to bring a candle and stand in a spot of their choosing. Over time, the available spaces filled up, and by the day of the action there were enough people to form a peace symbol.

This is a great example of using web technologies to achieve action in the real world. The iconic image was spread across the Hungarian newspapers and syndicated globally.

What other actions can be co-ordinated over the internet in this way?