The Frugal Flugel recommends Zipcar!
Zipcars can be found in cities across the USA and are a great option when you need a car for only a few hours.
There is a $25 one-time application fee to join and an annual fee of $50, but then you get dirt-cheap hourly rates.
In Chicago a Zipcar is $9 per hour; the same car in New York City is only $10 per hour.