One of the older travel sites around (http://www.airfarewatchdog.com/) but extremely useful when finding cheap airfare. Either alerts are pushed via email, you can browse on site or what's hunted above - their new iPhone app.
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Founded by an old friend, @georgehobica, he is the life force behind it. Extraordinarily well known throughout industry as an experienced travel writer and blogger.
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I was going to say "kinda like Farecast" (one of the coolest travel-related things from 10 years ago) but it looks like Microsoft / Bing Travel killed it back in April of this year. Sad. Glad to know there are quality alternatives.
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Spam Spam. Used to be cool but just takes you to other sites now. Kind of pointless to go to an airfare watch site that you cannot book a flight with these days. This site needs an update.
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I have tried this but I get more spam from them than useful stuff.
Polywork