Microsoft acquired FareCast and now provides it as part of it's Live suite of applications.
Also check out the FareCast blog (http://farecast.live.com/blog/), it has tons of useful information that can help you pick when you should buy tickets for your next trip.
Here is how the website describes itself:
Farecast offers unique features to help you Know When To Buy™, When To Fly™, and Where To Stay™—all based on science, not marketing. And now it's available from most major U.S. and Canadian cities to destinations worldwide. Our smart travel search also allows you to easily compare, sort, and narrow down flight and hotel results for hundreds of websites.
....you can find out if the lowest fare for a trip is rising or dropping over the next 7 days.
And here is some of the useful information that came out FareCast research (via Wired.com)
1. Common wisdom is wrong ...
The lowest price tends to hit between eight and two weeks before departure. Buying tickets farther in advance usually doesn't save money.
2. ... except when it's right
The rule fails during peak demand: Friday departures for spring break, and Sunday returns during the summer, Thanksgiving, and Christmas. For these, now is never too early.
3. When the price drops, jump
Fifty percent of reductions are gone in two days. If you see a tasty fare, snatch it up.
4. If prices seem high, hold off
Behavioral economists call it framing: If last year's $200 flight is now $250, you'll probably find that too dear and won't buy. Everyone else is thinking the same thing. So when airlines hike the price of a route, they often have to cut rates later to boost sales.
5. The day you fly matters
Used to be, you could count on a cheaper fare if you stayed over a Saturday night. But during spring break and summer, weekend trips are in high demand, so flights on Friday, Saturday, and Sunday can easily cost $50 more than those midweek.
6. So does the day you buy
Price drops usually come early in the week. So a ticket bought on Saturday might be cheaper the next Tuesday. That's particularly true outside the summer rush, making fall the best time for a last-minute getaway.
7. Markups vary by destination
Flights to Europe in July can be $350 higher than in May or September. If you want a summer vacation, domestic and Caribbean travel is cheaper to begin with and doesn't rise as high.
8. Stay an extra day
At the end of holidays, one more day at the destination can save you upwards of $100.