Farecast
This brand-new "airfare predication engine" calculates whether the lowest price on plane tickets — currently, only for trips out of Seattle or Boston — will rise or fall over the next week, to help you decide the right time to buy. (All major U.S. markets will be covered by the end of the year, according to a site spokesperson.) Choose from 120 destinations, and you'll get a fare history chart, price prediction and bottom-line tip (e.g., "Wait"). The site aggregates piles of airfare data to track price fluctuations and deliver its advice; it also reveals differences in fares according to time of day and other factors.