Hopper is a free app from an incorporated company of the same name. It provides airfare predictions, letting you select a time for a one- or two-way flight and letting you know when it will be best to actually book that flight. It also makes note of price-altering deals as they happen. When a flight is at its estimated lowest cost, the app also sends push notifications to inform its user!
A safe bet
In general, it's hard to completely predict, with perfect accuracy, how cheap or expensive a flight will be. But Hopper tries, and does admirably. Though you may spend a few dollars more than the absolute cheapest time, the app's algorithms are solid and will indicate the best time to book between now and any chosen flight. Naturally, the app does very well at things it can predict: sending notifications about prices or deals. Sadly, you can't check the app or get any information via browser, only via the app itself.
Overall, Hopper provides a service that's very valuable for frequent flyers or those with little money. Though there are no guarantees that the app's long-term predictions are completely true, our limited testing indicated that it was very reliable for tracking the costs of flights in the harshest months.