Now you can book and pay for plane tickets without leaving your computer. But are you getting the best deal?

All the travel Web sites work much the same way: after a brief registration process, you enter the reservations system—a sexier interface to the same computer networks used by travel agents—and input where and when you're traveling; back comes a list of possible flights and fares. (A few foreign and low-fare carriers—Southwest, most conspicuously—aren't on the networks.) As with non-virtual travel agents, there's no charge.

To test the on-line services, we tried booking a Valentine's Day getaway from New York to San Francisco. Beforehand, we called two travel agents and were quoted a round-trip fare of $299.36, requiring an early-Friday departure and late-Sunday return. Computers, we were to find out, don't necessarily indicate that sort of special deal. If the itinerary you select doesn't happen to fit a bargain schedule, you're out of luck. But even the sites that didn't at first give us the cheap fare did so when we tweaked our times to fit the fare's restrictions. The lesson: keep adjusting your travel times until you hit a good rate.

  • Microsoft Expedia
  • Initial quote: $437.55.
  • Upside: The wonderful "Fare Tracker" feature does the work for you, E-mailing you a weekly report on the lowest fares between three city pairs of your choice.
  • Downside: There are too many screens to go through before you finally reach the reservations interface.
  • Internet Travel Network
  • Initial quote: $332.09.
  • Upside: The simplest service, with a refreshingly clean design.
  • Downside: Site refers you to a local member travel agency rather than letting you book directly, a layer of bureaucracy that some might find annoying.
  • Travelocity
  • Initial quote: $299.36.
  • Upside: The "Three Best Itineraries" search engine builds an itinerary around the lowest price, a boon for flexible travelers.
  • Downside: A membership process that borders on invasive. Go for the hassle-free "guest" registration instead.
  • Reservations.com
  • Initial quote: $299.36.
  • Upside: The "Best Fare Finder" sniffs out the lowest-priced itinerary.
  • Downside: So why does it come at the end of the reservations process?
  • TravelWeb
  • Initial quote: $437.55.
  • Upside: The hotel reservations system is the Web's most innovative.
  • Downside: Flights are another story: no prices are quoted until you've chosen a complete itinerary, making TravelWeb essentially useless for bargain hunters.
  • PCTravel
  • Initial quote: $437.56.
  • Upside: A special travel club offers discounts on some business-class and unrestricted fares.
  • Downside: The hefty $10 ticket-delivery charge (other services deliver for free).

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