Rather than relying on algorithms as most other low-cost flight aggregators do, Airfarewatchdog actually utilizes a small staff to recognize and then post deals, both on its own site and also on Twitter and Facebook. As CEO George Hobica notes, "fares fluctuate like stocks," so you need to see it as soon as the price drops, which is why posting it to social networks is often better than the daily fare e-mails other sites offer. Airfarewatchdog tells you when fares between certain cities are on sale, which airlines, how many seats are available, advance purchase required, and any other quirky details. In short, they take the guesswork out of fare tracking.

COST: Free.