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."

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

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