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.
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