Yapta, is a tool to "tag" the flights you like while visiting many of the top travel aggreators. You can compare airline ticket prices either by entering your travel data by hand on their
website, or for batch tracking there is a browser add-on. Currently Yapta only supports IE, but a Firefox version is coming.
The cool thing about Yapta is its browser add-on injects code into pages when you browse sites like Expedia with links to “tag” the selected flight in Yapta. Once you’ve tagged a flight Yapta will track the price and watch for changes. If a price changes Yapta will send you an email notification.
The best part is the airlines “guaranteed airfare rule.” This rule says which says that if you buy a ticket directly from an airline and the price drops afterward, you’re eligible for a refund.
The airlines make the offer on the basis that almost no one is aware of it or none wants to hassle with gettting a refund. The great thing Yapta handles the tracking for you but you still need to contact the airline directly.