We all know that there are airfare search sites such as Expedia, Priceline, and Cheapair.com. But then there's a whole different animal: the airfare listing and alert site.

Sites in this latter category, which is growing at a rapid clip, may include a search component, but they also provide a list of low fares and/or alerts about such fares, either delivered by e-mail or posted online. They're ideal for people who are just looking for somewhere cheap to visit, or who are hoping to travel on specific routes but want to buy when fares are at their lowest. In alphabetical order, here are 10 of the best.

Airfarewatchdog.com: This is the only site in the group that lists and compares fares on Southwest Airlines (now the largest domestic carrier based on passengers boarded, but one that refuses to pay sites to send it traffic), as well as those on low cost carriers such as Allegiant and Skybus, which sell fares only on their own Web sites. Whereas the other sites listed here use computer programs to evaluate fares, Airfarewatchdog uses people, who actually test if seats are available at the fares listed (sometimes airlines file fares for which very few or no seats at all are available, which is very naughty of them).

However, Airfarewatchdog.com does not offer individual city pair alerts (although this is in the works); rather, if you sign up for alerts you'll get a list of fares from your chosen airports that the staff believes are unusually good deals. And the site doesn't monitor nearly as many routes and fares as some of the other sites on this list. Uniquely, however, each list of fares from a given airport also includes fares from nearby alternate airports on one page, and you can put your alerts on "vacation hold" for up to a year.

(Disclaimer: George Hobica, the author of this piece, writes for Airfarewatchdog.com.)

Bestfares.com: Earlier this year, Best Fares underwent a design change and now its fare listing area most nearly resembles Fare Compare (below) more than any other site. On the home page, you'll see a box that reads "Type your Departure City here" and a "go" button. You'll see a listing of fares to hundreds of airports from your city. These fares include some but not all taxes, and you'll also see "member only fares" listed along with "published" fares.

The member fares are typically specially negotiated fares — sometimes called "consolidator" fares — that are offered from time to time to various travel agents. You cannot book fares on Bestfares.com; rather you have to call a toll-free number to complete your purchase. But like Farecompare.com below, Bestfares.com provides a good barometer of where fares are at any given moment, and the fares are updated throughout the day.

Farecompare.com: This is the new breed of automated fare comparison and alert systems. You can sign up for as many individual city pair alerts as you like, and you can also choose to see only those fares that have hit their historical low point or that have gone down by a lesser amount. You can also see a list of all fares from the airport(s) of your choice, and you can specify either domestic or first class fares. The alert sign up procedure is very clear and easy to use. The "Getaway Map" lets you browse a map from your chosen airport showing the lowest fares to various domestic and Canadian destinations, and you can refresh the map based on the exact week that you'd like to travel. Fares are listed from lowest to highest.

Farecompare gets its data from a company called ATPCO, which processes fares from the airline computer systems and delivers them to travel agencies and what are known in the industry as Global Distribution Systems (GDS's) such as Sabre, Galileo, and Apollo. Domestic fares are updated by ATPCO (and thus by Farecompare) three times a day during the week and once a day on weekends.

Farecast.com: This site is best known for predicting where an airfare on a particular route is going to go (up, down, or stay the same) over a period of time. But recently it, too, got into the fare listing game. From the home page, go to the "Airline Ticket Deals" section and you'll see a select, but not very extensive, list of fares from the airport of your choice. Unlike Farecompare, Farecast uses airfare data from Cambridge, MA-based ITA Software, which in our experience is more accurate than ATPCO's data. Farecast's fares show all taxes up front, but Southwest, Allegiant, Skybus and a few other airlines' fares are not shown.

Internet Airfares: Long before there was a Farecast or a Farecompare, there was Internet Airfares, an easy to use if limited site that shows the best fare reductions from various cities. It's not pretty, and only about 50 departure airports in the US and Canada are covered, but it has its charms. Again, no Southwest Airlines and this site doesn't send out email alerts.

Orbitz.com: This well-known online travel agency is best known for its fare search capabilities (like Farecast, it runs on ITA Software). But Orbitz also offers fare alerts with its "Deal Detector" (find it under the heading in the lower left of the home page under "Orbitz deals — delivered to you"). Once there, fill in the departure and arrival airports (you can also opt to get alerts for cities within 80 miles of your main choice), choose whether it's a weekend trip or not, specify your dates of travel, and decide whether you're flexible plus or minus 1 to 3 days. Then set your target price and whether you prefer nonstops. You can only sign up for one deal at a time, and again, Orbitz doesn't list every possible airline.

Sidestep.com: In early September, Sidestep added a unique fare alert product that allows you to track a specific airfare. To use this service, you need to first search for a fare between two cities. Once that's done you'll see the option to track the fare, and you can choose to track either nonstop or connecting/direct flights over a flexible travel date period of between 7 and 30 days in either direction of the dates for which you originally searched, which is a unique feature. Sidestep does not (yet anyway) offer a fare listing service.

Smartertravel.com: This granddaddy of airfare information sites recently added a "deals at a glance" area under the "deals" tab that shows a selection of advertised fare deals. But the site is perhaps best known for its extensive and easily sorted reporting on last minute weekend deals, which are sent out by email or can be seen online.

Travelocity.com: Travelocity, as far as we know, was the first travel agency site to offer a fare watch system. Once upon a time, it tracked both international and domestic fares, but now only covers domestic/Canadian deals. To use it, go to the "Flights" tab and then to "Low Fare Alert" and then click on "Get email alerts" in the upper righthand corner under "Tools". You can sign up for alerts on up to five routes. On the Low Fare Alert page you'll also see a list of fares, from any given city, that have gone down in price from the previous day. Travelocity searches fares just once a day, however, compared to Farecompare's three times a day Monday-Friday.

Yapta.com: The newest entrant in this burgeoning field. There are two things you can do here: first, if you've already bought a fare, you can enter the flight locator number and then you'll get an alert if the fare has gone down. (Some airlines will refund the entire fare difference in the form of a voucher good for future travel within a year; others deduct $25 to over $300, depending on the fare and route.) But there's also a "Tagger" feature where you can track fares on individual flights, not just routes between two cities.

So, for example, if you really like to take United's morning nonstop between New York and LA, you can track just that flight to see if the price drops to a level you specify, and you'll also get an alert if the price drops between the time you buy and when you fly. Yapta requires that you download an applet, and only works with Internet Explorer 6 or 7, which means it doesn't work on Macs, although a Mac/Firefox version is in the works. And it does not (yet) track fares on all airlines — just 11 so far, and as usual Southwest isn't one of them.