Farecast got bought by Microsoft and was used in Bing Travel, but it seems to be gone now. According to Wikipedia (http://en.wikipedia.org/wiki/Bing_Travel), "As of January 2014, the fare prediction feature has been removed."
Farecast predicts airfare price movement over the next 7 days for a set itinerary (dates, origin/destination). This service tells you when you should go on vacation, not when you should buy your airfare ticket.
