Zipcar is a nice evolution of car rentals.  They refer to themselves as a car sharing club and not a rental car company.  How it works is zipcars are scattered around a few select cities (New York, Toronto, San Francisco and a few others) in conveniently located parking spots.  Members can just reserve cars online and then go pick them up at the local parking spot using their membership card to open the doors.  There are a few things that are really cool about this.  The first is that you can rent by the day, the hour, or as part of a multi-use package depending on your needs and all the rates are pretty good.  Secondly, the cost includes gas, insurance and parking, which is awesome.  Also, they have some interesting cars to rent including Minis, BMWs and hybrids.  Lastly, because the cars are just in parking spots, you can pick them up whenever you want, 24/7.  This is a really great deal for people who live in cities like New York where you don't really need a car but it is nice to have one once in a while.