The result is that the five largest online travel services represent 57% of the online travel market, PhoCusWright says. So even if sites are launching, the big ones are buying up what they need to ensure that each spot in the travel market is covered.

Those who believe travel sites are strong businesses compare travel with the financial-services market, which also hasn't been decimated the way many consumer services have. As Expedia's LeVine points out, travel isn't tangible and doesn't require physical inventory control or shipping and handling. There's also an increasing amount of international travel, which is growing at more than 5% annually, according to the International Air Transport Association. In addition, the travel industry is highly stratified, with airlines, cruise lines, car-rental companies, hotels, individual reservation systems, travel agents, and consolidators fighting for a share of every dollar of revenue collected. The industry's conclusion: The Internet is the ideal platform to exploit each of these conditions and increase a robust business.

The rise of travel sites is also linked to excess inventory. Surplus levels change daily, if not hourly, and that continues to attract participants adept at sniffing out commission opportunities.

"The airlines fly 500,000 to 700,000 empty seats per day, depending on which group you talk to. And there are as many as 1 million empty hotel rooms in the United States per night," says a spokesman for Priceline.com Inc., a reverse-auction site that has diversified into travel services. "If you're wondering why there are so many sites out there, it's because the airlines have a big challenge: putting people in those empty seats." The same argument can be extended to rental-car companies and hotel rooms.

But there's growth in other areas, as well. Just last year, almost 15 million households were projected to book leisure travel online, generating about $12.2 billion in sales. This year, 18.9 million households will buy travel online and spend $16.7 billion, says Henry Harteveldt, a senior analyst at Forrester Research. That's just in the United States; the global travel market, both off-and online and consumer and business, will generate $43 trillion.

"Any travel company worth its salt has its own Web site--airlines have been most successful because they understand customer behavior and see their sites as outlets for excess inventory," Harteveldt says. "The Internet is no longer mysterious and demanding. Users know there are deals out there. And they like it for three reasons: convenience, prices, and the control it gives them."

Airline tickets are the most commonly purchased travel product online. But because online commissions are only $10 a ticket, travel sites are looking to acquire other sites or partner to offer services with better margins, including lodging, car rentals, or specialty travel, such as cruises or exotic adventure tours.

The polarization of travel sites to the very large and very small forces operators to understand how and where they want to participate in the online travel market. Some want to operate in the background and carve out a more margin-rich business than selling last-minute seats to Paris. Others are willing to spend to build their brand name and drive travelers to their site en masse.

GetThere Inc., acquired by Sabre last August, offers business-to-business travel software. "It takes a large investment to establish brand awareness and to invest in the underlying technology platform," says Jeff Palmer, GetThere's chief operating officer. "We had the choice to invest in brand awareness or technology. We chose technology--the technology to power the functions for the established brands." As a result, the American Express travel agencies, United.com, and company intranets such as Chevron, Nike, and Kodak all use GetThere.com.

The world looks a little different to the mass-market sites. "The primary driver of our business is the customer shift from offline to online, not the growth in the underlying travel market," says Expedia's LeVine. Growth in ticket sales has been as high as 7% annually, but Expedia's revenue from airline ticket commissions is expected to grow 65%. Expedia has helped that growth along through acquisitions, partnering, and co-branding. "There are so many costs--the cost of entry and sustaining the businesses--that you need to 'go big' early on," LeVine says.

Whether size will help travel sites during a period of economic uncertainty remains unclear. Analysts predict many sites will attempt to distinguish themselves with rich media, permitting site visitors to "tour" a hotel room or see the view from a cruise ship's private balcony. Other site operators may make greater use of wireless communication to alert travelers to special fares or airport delays. Amadeus, Sabre, and other large reservation systems will try to sell large business accounts on the benefits of attaching directly to their systems, circumventing travel agents and protecting the account from competitors' overtures.

Despite the slowing economy, online travel services are unlikely to face the same fate as Web sales of dog food or Viagra. "Online travel has never lived through a recession, but it will survive because it offers a convenient, efficient, and cost-effective way to buy and sell travel," says Philip Wolf, president of PhoCusWright, adding that travel is the single largest, repeat discretionary expense. "Other things cost more," he says, "but how many times do we buy a house vs. fly on a plane? Consumers of all types and nationalities continue to defy skeptics with an insatiable appetite for anything and everything online travel."