For the urban dweller that might only need a car every few days, or suddenly find their car out of commission, car-sharing services have become an increasingly popular alternative to car ownership.

Car sharing is a little like renting a car, but once you have signed up the process is much easier. The cars can be rented by the hour for rates as low as $8 an hour.

Two of the largest sharing providers, Zipcar and Flexcar, agreed to merge last year and operate under the Zipcar brand. Zipcar started doing business in major cities like New York, Boston and Washington, D.C. in 2000. It recently expanded operations to Pittsburgh and Philadelphia and operates rental sites in major cities all around the country. Zipcar now has locations in over 20 major cities and claims on its Web site to have over 20,000 members.

Here’s how it works. After signing up and paying an application and membership fee, members receive a card and are able to make reservations for cars 24 hours a day. Once the reservation is made online, members take their card to one of several locations around the city where Zipcars are parked.

Waving the card in front of a sensor on the windshield opens the car and the keys are located inside. There are no lines or paperwork involved. Gas is included in the reservation. There is a gas card located in the car and members are allowed to use the card to fill up. Insurance is also included, as is 180 free miles.

Zipcar also claims to be an eco-friendly alternative to automobile ownership.

"Each Zipcar put on the road replaces 15 personally owned vehicles,” said Steve Bishop, General Manager of Zipcar Chicago, in a press release. “Members give up their private cars, drive less, and walk, bike and use public transit more often. It doesn't get much greener than that."

But not everyone is eligible to rent and drive a Zipcar. Aside from needing a legal driver’s license, members must have had no more than two incidents involving moving violation plus accidents in the past three years, and no more than one incident in the last 18 months. They are also required to have no alcohol violations in the past seven years.