If you don’t know what ZipCar is then you can read more about it here. In short it is a car sharing rental company for large cities. Personally I just signed up because I do not have a car since I live in Manhattan. It is a great service to rent a car by the hour or pretty cheaply by the day. They have cars in most cities which you can check out on their website.
When I signed up I found the downside to be that there is a $50 yearly membership fee and a $25 one time application fee. What I found that finally made me seal the deal was a coupon on retailmenot.com for $100 driving credit when you sign up and use the coupon code DCOFFICE100
It is not that difficult to find a $50 off coupon but this one paid for the $25 activation + $50 yearly fee with $25 of free driving left to spare. While this is not a discount it is the best deal I have found so far. Since I did it two weeks ago and already used the driving credit I can confirm that it works.
Just thought I would share my 2 cents . Let me know if it works for you!