Zipcar is pretty good but not

very cheap.
I used Zipcar for couple of years. I work at an University and I got a the membership for free (usually it costs about $50 annually). Plus we have discounted rate for $6/hour (regular cost $8).
Zipcar charges base on Hourly and Daily. Each car has it's fixed spot that means you need to pick it up and return at same location. There is also an overnight charges.
They have a lot of fun cars to drive, I drove the Soul, Smart, new Insight, and Mini Cooper. Anyways, it is very convenience and useful in cities.
Hope this helps.