We just heard from Aaron at AT:NY that Zipcar is running a special promotion in Chicago this summer called the Low Car Diet. If you agree to forgo using your car for 1 month (7/21 - 8/15) and use a combination of Zipcars and other ways to get around (walking, biking, public transit) they will give you a free 1 year membership, a loaded transit pass and a driving time credit. What a deal!
I've been a Zipcar user for just about 2 years now (and car free for more than 8) so I know personally all about how easy and great it is to get around Chicago without owning a car - I hope LOTS of folks take this challenge and find out for themselves that it is a good way to go.
Available in Chicago and various other cities, check with Zipcar for all the details.