FareCast was purchased and incorporated into bing travel, re-named Price Predictor (description here). The general search engine for bing is so-so, but better to stick with Kayak, from which much of bing’s interface is borrowed.
Price Predictor displays two months of price history for specific itineraries for certain popular routes (mostly US domestic but some international). It gives a handy chart with volatility and makes a prediction about when to buy with confidence estimates. The charts usefully show the predictable pattern for many routes, often prices dropping on Tuesdays, then climbing to weekend and Monday highs.
The big downside is that the Price Predictor results cannot be refined by time of day, number of stops or carrier. For one route The Rapid Traveler frequents, Spirit consistently undercuts the competition with one-stop itineraries which skews the data but is not an option under consideration, so the prediction is virtually useless. Where mainline carriers with nonstop flights charge a predictable premium over the low-ball it is possible to back out some information but that takes a lot of additional research.
Verdict: great concept but of limited appeal except for situations where all flight options are acceptable. Good for some basic research on unfamiliar routes. But the main use seems to be kick yourself for not getting a better fare a few days ago. File in the ‘secondary’ folder of your travel bookmarks.
(Disclaimer: The Rapid Traveler has no financial relationship with bing Travel or Kayak and will not receive any commission or other consideration for this review.)