The open streets movement aims to give public roads back to everybody, if only for for a few days each year. We don't realize how much city space is inaccessible to non-drivers until we block a road for a Sunday and see what people do with it: Walking, biking, dancing, aerobics, games, socializing etc. People having a good time, getting some sun and exercise, and rediscovering their city from a new angle. Definitely a good way to create a sense of community!
The Open Streets Project is a great resource if you're interested in learning more, and especially if you want to get a ciclovia going in your city!
You can see a map and list of initiatives around the US and Canada here.
Bogota's Ciclovia has been an inspiration to the rest of the world when it comes to these types of events, and it's easy to understand why. Check out this video from 2007 that shows how they did it back then (they've improved since, no doubt):
Portland had its first ciclovia event in 2008.