A growing arsenal of "aggregator" Web sites perform as the K-9 Corps of online air travel. These sites simultaneously search multiple travel provider sites to sniff out the lowest fares available for a particular itinerary, time frame and class of service. Some even allow you to target your search to airline, total travel time, number of stops, routing and other parameters.

The most useful of these aggregator sites will save you hours of research by providing within seconds the best fares for your destination that can be found on the Internet.

Although the majority of these search engines don't sell tickets directly, instead collecting fees for referrals to travel providers, there could be issues of bias and market saturation. Some sites have airline sponsors whose flights tend to be listed ahead of others, whether or not the fares are the lowest. And because each aggregator searches a specific number and kind of travel provider sites, the fare results will typically vary from one aggregator to another. You will have to check out two or three of these search engines to ensure that you're presented with all the applicable fares available.

I have investigated more than a dozen well-reputed travel search engines online and have narrowed the A-list to one all-purpose strictly informational site and three sites which search and then sell directly or by referral. Although several aggregator sites offer searches for hotels, rental cars and even cruises, I'll focus solely on the top sites for airfares.

Note: The Big 3 online travel sites — Expedia.com, Orbitz.com and Travelocity .com — conduct business as full-service agencies to book and issue tickets for all kinds of travel products. Although I have included them in the fare comparison, they are not technically aggregators.

First, you should know about a brilliant tool for determining benchmark airfares. Every single day, I log on to ITA Software's Fare Shopping Engine, matrix.itasoftware.com, an endlessly rewarding beta site created by MIT grads. Their QPX software searches thoroughly the published fares of nearly every airline on the planet, then presents them in a grid that can be configured to the options you desire. You may request, in ascending/descending order, such features as price, airline, departure times, number of stops, routing, total travel time and more. This is the only search engine that permits so many consumer options in determining the lowest to highest of published fares, but it does not search privately negotiated or wholesale fares.

According to the site, "We can find and organize more options than a traditional travel agent or travel Web site. QPX provides all of the information required to book and ticket any itinerary directly in a carrier's inventory system or in a travel agent's Global Distribution System." Although ITA sells nothing, the real beauty is that your finely tuned itinerary and detailed booking codes and calculation of fare basis can be e-mailed or faxed directly through the site to your travel agent for immediate ticketing.

Another favorite, genuinely useful site is Kayak.com, a meta-search engine that scours more than 120 travel Web sites worldwide in real time. It searches all published airfares plus some private, proprietary fares available only through discounters and consolidators.

Kayak is not a travel agency; it doesn't sell tickets but directs consumers to travel sites for purchasing online. This site, 2006 winner of Forbes Best of the Web, states: "Our way of helping you plan your travel gives you a more comprehensive list of your travel options."

Cfares.com is one of my new best friends. This unique airfare search site "delivers a 360-degree view of all airfares in the market, including deals you won't find anywhere else." Cfares searches rates directly from the airlines, from major travel Web sites and from industry wholesalers. Standard Gold membership is free. Platinum membership at $50 per year offers, additionally offering "customized, below-market fares via on-the-spot discounts." This site is particularly adept at finding the lowest available business and first-class fares. When you want to make a purchase, Cfares will link you directly to the online vendor.

A compelling feature of this site is the exclusive cAgent service that allows consumers to detail a specific international itinerary and name a desired price. The search engine will scour "deeply discounted inventory on a continuous basis for up to a week" then e-mail the potential purchaser with results. Cfares will hold the reservation and fare for 24 hours.

Rounding out my choice of the top three aggregators is Qixo.com, the only travel search engine site that sells directly to consumers. Using advanced proprietary search and auto-booking technology, Qixo consistently offers some of the lowest coach fares available from any source. Unbeatable for the price-driven traveler on a roundtrip itinerary. However, its utility is limited by not offering such desirable options as business or first-class tickets and multicity itineraries.

Irene Croft Jr. of Kailua, Kona, is a travel writer and 40-year veteran globetrotter. Her column is published in this section every other week.