Zipcar, Getaround, Uber: Are rental-car alternatives for you?

Visiting San Francisco? You could zip around town in style with a rented Porsche Cayenne, an Audi A3, or an all-electric Tesla Model S.

These listings aren't from Hertz, Enterprise, or the other major rental agencies. They were posted to Getaround.com by individuals willing to lend their cars to total strangers at an hourly rate.

Car-sharing programs took a sharp turn toward legitimacy this year. Avis bought vehicle-sharing pioneer Zipcar for $500 million. Taxi-on-demand service Uber reportedly passed $1 billion in bookings this fall. And peer-to-peer programs, such as Getaround, expanded nationwide.

These novel services – all fueled by smart online applications – come with different perks and costs. But each boasts some degree of flexibility, low costs, and green credentials. Here's a breakdown of current offerings:

Membership programs

Zipcar designed its program around short trips. The company, now 13 years old, knew that city dwellers and students didn't need cars full time, but they could use one for the occasional drive to the supermarket, a ski slope, or Ikea.

Customers can reserve a car online, unlock the vehicle with a simple tap of their membership card, and hit the road. The hourly rate (starting at $9.75) covers insurance and gas.

With the success of Zipcar, which now has 11,000 vehicles parked in 18 American cities, came several competitors. Daimler launched Car2Go in eight cities, and BMW started DriveNow in San Francisco. While Zipcar charges annual membership dues ($60, plus a $25 registration fee), Car2Go and DriveNow ask for only a one-time fee of about $35.

Car sharing comes with substantial green credentials. Each vehicle involved in a car-sharing program along the lines of Zipcar results in nine to 13 fewer cars on the road, according to a 2011 report from the University of California Transportation Center. Zipcars also tend to be newer and get better gas mileage than the cars Zipcar clients might use otherwise.

Traditional agencies

Membership programs are priced by the hour, making weekend getaways costly. For longer trips, you might be better off with a traditional rental agency.

There is also a distinct advantage to having a formal agency check out each car after every reservation. With car-sharing programs, you run the risk that the guy who reserved a car before you shows up late, makes a mess of the vehicle, or leaves you with an empty tank.

Peer-to-peer car sharing

These programs rely on the trust of neighbors. Rather than buy a fleet of vehicles, Getaround and RelayRides offer a way for people to rent their personal cars. Families can make a little money off a second car or from a vehicle that would otherwise sit idle.

Two start-ups have brought this business model to airports. Rather than deal with long-term parking fees, owners can rent their cars to FlightCar or Hubber. The companies will provide a free shuttle to the airport and seek out a vacationer in need of a car.

These services forgo any membership fees and often allow vehicle owners to set their own prices. The companies also insure the cars in exchange for a share of the proceeds. However, some states have questioned the coverage offered by these companies. RelayRides suspended service in New York after officials there said that "the company's insurance is illegal and inadequate."

Taxis on demand

Uber applies the Zipcar model to taxis. Its smart phone app lets you hail a cab from any of the 62 cities it services worldwide. Uber uses your phone's satellite positioning to figure out which drivers are nearby and shows you an image of who will pick you up and how far away they are.

An estimated fare appears on the phone before the car arrives. Prices vary based on distance and the quality of the car. Customers pay through their phones and may automatically split the cost among several passengers.

For more on how technology intersects daily life, follow Chris on Twitter @venturenaut.