Several websites are offering travelers the ability to search over time for the best air fares rather than at a single point in time.
With fares changing many times a day, services like Travelocity's FareWatchers, Orbitz's Deal Detector, and the new TripStalker try to give travelers the tools to find the best deal whenever it comes along.
FareWatchers has been around the longest. It launched in 1997, one year after the online travel agency started. Today, the service has more than 5 million subscribers and sends more than 300,000 e-mails a day notifying members of air fare changes that interest them.
FareWatchers offers two types of e-mail alerts: one when Travelocity's lowest fare between two targeted cities changes up or down by more than $25 and the other when the fare falls below a threshold set by the subscriber.
''People know that airline prices tend to fluctuate from one day to the next," said Joel Frey, a Travelocity spokesman. ''The service helps keep them informed and helps them get a good deal. It drives a lot of sales for us."
Orbitz's Deal Detector is similar. Users set a target price for a trip on a certain date between two cities and Orbitz e-mails them when a match is found. Searches can be modified to include nearby airports and to add extra days to the search parameters.
TripStalker, a stand-alone company out of Austin, Texas, is the latest company to enter the business of what might be called ''ongoing search." It hunts not only for air fares, but also hotel rooms and rental cars. It's not an open-ended search, but instead targeted at people planning an upcoming trip on a specific date or dates.
''TripStalker is really a niche application, trying to serve a specific need of people whose time is constrained," said J.P. Maxwell, president of the company, which has about 1,000 subscribers.
The other key difference between TripStalker and the other ongoing search functions is that users of TripStalker download the company's software to their own computers and plug in where they want to go, when they want to go, and how much they want to pay. TripStalker constantly searches for fares that meet the search parameters and notifies the subscriber when the target fare is matched or beaten. Subscribers receive notification on their desktop, by e-mail, or by text messaging to a cellphone.
Many consumers are wary of downloading software to their own computers, but Maxwell said the approach has a number of advantages. He said users can update their search or check the latest results with a click on their toolbar rather than logging on to a separate website.
Eventually, users also will be able to put the power of their own computer to work for them, Maxwell said. Currently, TripStalker software reaches out to the company's computer servers and searches for deals using many of the same reservation systems used by the online travel agencies. In the coming months, Maxwell said, the software will be upgraded to enable the subscriber's computer to reach out directly to the websites of airlines like Southwest and JetBlue that don't participate in the reservation systems.
''We're doing it with the subscriber's permission and only for the stalks that he is interested in," Maxwell said, adding that the legal logistics still have to be worked out.
TripStalker currently earns money when users find a fare they want and click through to buy it. If the software is upgraded to offer direct searches of airline websites, Maxwell said, his company would either have to solicit advertising or charge extra for that service. TripStalker users now pay nothing for the software.
Contact Bruce Mohl at mohl@globe.com. 
