When winter storms struck the Midwest and East Coast this month, savvy fliers didn't turn just to the airlines for help. Thousands went to privately run Web sites to check on delays and to track the flight paths of the first jets to escape the city's snowbound airport.

Frequent fliers mostly find the sites helpful in planning trips and avoiding delays. But those who follow the airline industry say they expect the sites to become more popular as fliers become frustrated by increasing flight delays and what many think is unreliable information supplied by the airlines.

"Passengers are getting savvier and getting less dependent on the airlines," said Michael MacNair, chief executive of MacNair Travel Management and author of "Smooth Landings," a book on managing business travel. "The airlines don't have as many people available to track flights and provide that information to travelers. So they have to do it themselves, and these are some of the tools where they can get the information they need."

All the sites are free and are supported by advertising and sales of data to companies such as limousine services.

The sites, which rely on government radar and flight plan data, track planes from takeoff to touchdown. They show a map that plots a plane's location, its path and information about its speed, altitude and estimated arrival time.

To track a flight on the sites, users need the airline and flight number or the flight's destination and arrival airports. Some of the sites publish maps depicting air traffic near airports. Others will send users e-mail or text messages about a plane's takeoff or landing.

A plane's location on a site's map is not precise enough to compromise security, the companies say.

Many frequent fliers agree that one of the best sites is run by FlightAware (http://www.flightaware.com), a Houston company that started operation in 2005. The site has about 1 million unique visitors each month and saw a 34 percent increase in visits during the Denver snowstorm on Dec. 21 and 22, chief executive Daniel Baker said.

Fliers say they have found the site useful, especially to track planes carrying their friends or relatives. David Gilmore, a computer researcher in Portland, Ore., used FlightAware a few months ago when his parents were flying in to visit him.

The United Airlines Web site said the plane had left its gate in Chicago, but its estimated arrival time changed by an hour or so every few minutes. Gilmore suspected that the flight hadn't taken off yet. He logged on to FlightAware, confirmed his suspicion and monitored the site until it reported that the plane was in the air.

"United's site eventually caught up, but it took a while," he said.

By studying the previous tracks of a flight he would like to take, Gilmore has used the site to help him select his seat. He said he didn't want to miss pretty scenery by sitting on the wrong side of the plane.

Other fliers said they use the site to figure out whether an airport is experiencing delays so they can alter travel plans to avoid the snarls. Some have even found ways to track the jet that they will be getting on later in the day. This gives them a better sense of whether they will depart on time, they said.

Frequent fliers who were asked about tracking sites also praised these three: Flight Explorer (http://www.flightexplorer.com), FlightView (http://www.flightview.com) and FlightStats (http://www.flightstats.com).

FlightStats provides easy-to-use graphics that show the on-time performance of flights. It will even rank the on-time performance of all flights between selected airports and send you text-message alerts about a flight's departure and arrival.

FlightView sells a $19.95 screen saver that displays constantly updated flight tracks of departures and arrivals across the country or at airports.

The sites, including FlightAware, do not generally provide all the information that fliers and their relatives might need to meet people at airports. They track flights only after they are in the air and do not say why a flight is delayed. Most of the sites don't provide gate information.