When Che Carsner wanted to find discount airline tickets for his parents from the Miami area to New York, he knew where to look. The Manhattan real estate agent logged onto Kayak.com, a new online travel aggregator, and typed in some dates. Within seconds, dozens of options appeared. Among them: a $140 round-trip fare from Fort Lauderdale, Fla., to LaGuardia airport on US Airways, which he booked.

"I might eventually have found that fare elsewhere if I had done a lot more clicking," Carsner said. "But by putting in the right data, it came up instantly. And the interface [on Kayak] was a lot smoother than anything else I have come across on the Internet." Finds like that have turned Carsner into a devotee of aggregators, a growing class of search engines that scan the Web for bargains on airfares, hotels, car rentals and other travel services.

He's not alone. Between December 2003 and October 2004, the number of users who downloaded software from SideStep.com, the most established travel aggregator, rose from 3.25 million to 4.75 million, according to Phil Carpenter, SideStep's vice president of corporate marketing.

Aggregators are Web sites that function like travel-themed versions of Google. Besides Kayak, the major players are SideStep, Yahoo's Farechase.com, Cheapflights.com, Mobissimo.com, Qixo.com and SuperSearch (a subsidiary of the discount online agency Travelzoo.com). When a user logs on and inserts specifications for flights, hotel rooms, rental cars or other travel services, the sites scan their databases for possible matches. Most aggregators have ties to a much broader network of links than more familiar online agencies, so they offer users a wider variety of options. While the popular Orbitz has flight data from more than 455 airlines, SideStep includes 585 carriers in its network. Kayak searches 550 airlines, including fares from other booking sites. The newest of the group, this Connecticut company was started up a few weeks ago by veterans of Orbitz and other major online travel agencies.

Several other agencies useful for travelers offer more limited services. One is Travelaxe.com, which searches only hotels. Another is BookingBuddy.com, which provides links to Orbitz and the other familiar online agencies.

The aggregator offers a list of airlines, hotels or other providers, along with the providers' toll-free numbers or links to their Web sites. Bookings cannot be made on most aggregator sites; they act as search tools or referral services to airlines, hotels or online travel agencies. The customer can then select the best match and make reservations directly on the Internet or by phone with the provider.

For example, when we logged onto Farechase.com looking for airfares from D.C. to Hong Kong, the site listed fares offered by United, Continental, US Airways, Cathay Pacific and American. To find out the details or make a reservation, it was necessary to click on the airlines' Web sites and book through them. The other major aggregators function similarly.

Svetlozar Nestorov, president of technology at California-based Mobissimo, explained the difference between aggregators and such better-known online agencies as Expedia, Orbitz and Travelocity. "They are companies that are trying to get you to book a ticket, a room or a car through them," he said. "We're neutral sources that search out all the information and present it so you can make the choice of which option best suits your needs."

SideStep's Carpenter was more blunt: "The agencies want to convince customers toward buying a particular product. We're trying to give them a sense of the range of options out there. We think they should have control over booking the options they prefer."

Experts predict a surge in aggregator use as more travelers turn to the Web to book trips and the technology becomes more refined. "They have already staked out their places and will certainly gain popularity as consumers develop more confidence in what they're offering," said Lorraine Sileo, an analyst with PhoCusWright, a Connecticut travel research firm.

Besides offering a wider selection of options, aggregators differ from online travel agencies in several ways. Many of them include no-frills budget airlines like Southwest, JetBlue and Canadian-owned Jetsgo, companies not listed on Orbitz or other major online booking sites. And, with the exception of Qixo, which charges a $20 booking fee, aggregator use is free. Instead of booking fees, they receive revenues from advertisers. Some also get a small pay-per-click fee when they refer a site and a user clicks on it.

As travel tools, aggregators are far from perfect. SideStep's software is time-consuming to download. (While the download is necessary for airline searches, hotels and car rentals can be scanned through a Sidestep Web site that is not necessary to download.) Once installed in a computer, SideStep is ubiquitous, popping up whenever a user starts to look for rates on other sites. While some consumers like the instant comparisons it provides, others find it annoying.

Other sites, like Qixo, are not particularly user-friendly; searches on it can take several minutes to complete.

And in spite of their broad Web searches, aggregators don't always offer the cheapest airfares.

To test the system, we searched the major players for a round-trip ticket from Dulles to Madrid for an arbitrarily chosen week in November. The fares ranged wide. (See chart on Page P1 for details.) The cheapest appeared on a link offered by Cheapflights.com for an Alitalia flight via Milan for $389; Mobissimo offered the same flight for $401. The most expensive was a British Airways flight SideStep found for $514 via London.

Broadening our search to non-aggregator sources, we found a flight for $416.68 on the Web site for Iberia, the Spanish airline. None of the aggregators had listed it. When we tried the same search a couple of days later, however, the Iberia fare popped up on SideStep. Conclusion: Shop around.

In some cases, aggregators work best as informational sources that give a sense of the range of rates available for a particular destination. That's how Joe Ehrlich, a San Francisco backpacker who frequently scours the Web for deals, uses them. In a hunt not long ago for a ticket from San Francisco to London, he logged onto Mobissimo. It was a useful tool to get an idea of the array of fare prices, he said, but after some comparison shopping, he found a better deal on Travelocity.

As the aggregators gain stature, the tug between them and online agencies for customers is becoming fierce. Kayak initially used Expedia as one of the sources where it searched for fares, but last month Expedia said it did not have an agreement with Kayak and asked to be removed. "We believe that Expedia and all of the other agencies will agree to be listed once they see that we're here to stay and are offering a service that will help them," said Dana Galin, vice president of communications at Kayak.

And last month, Orbitz announced that it could guarantee the lowest fares on the Web for all the airlines included in its database. If a customer books with Orbitz and then finds a published airfare at another online site for the same airline and itinerary that is at least $5 less, the company says it will give the traveler a $50 coupon good for future travel.

Hugo Burge, chief executive officer of Cheapflights.com, downplayed the guarantee, saying it has so many conditions that customers will have a hard time cashing in. He recommended that travelers shop around for deals. Indeed, in our test for Madrid fares, the cheapest we found on Orbitz -- $508 on US Airways and Lufthansa -- was beat by many of the aggregators.

Aggregators lead to the lowest price enough of the time that they are worth checking out. They might also lead to the most convenient connection. "We're finding that travelers are not always looking for the lowest price but the best option given the kind of parameters they are operating under," said Beatrice Tarka, CEO of Mobissimo. "And that's what we want to lead them to, the best deal available, taking into account all their needs and wishes."

INSIDE

How the aggregators compare and fare. Page 6.