Farecompare.com is a good first stop for leisure travelers with flexible travel dates or locations. You may want to visit the Caribbean sometime this summer, for example, without having a particular city in mind.

Farecast.com helps travelers time the ticket purchase once they've decided on a destination. The site went live in 2006 with information for two departure airports and now services more than 75 locations.

Yapta.com, launched this year, watches fares for specific flights. A handful of airlines, like United and American, will sometimes issue credits if the fare drops after the ticket purchase.

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