Here is a link to join zipcar for free the first year. The $25 application fee and $50 first year membership fee are waived when applying through this link.
You can join using the link from Boston, but your membership is good wherever there are zipcars.
The concept is relatively new in the USA but spreading to other cities. The current Boston rate is $9 per hour and New York is $10 per hour.
Check it out.