Trying to figure out how to afford that snowboarding trip on a student's budget? Need to go home and finally cut it off with that high-school girlfriend and hate to pay more than you have to? The Airfare Watchdog is a great service - one I'm surprised we haven't seen before. Anyone who's followed the daily fluctuations of ticket prices knows the unpredictable crap-shoot it is trying to guess when to buy. With Airfare Watchdog you can sign-up for daily or weekly notifications, complete with suggestions on hidden fares and other offers. Unfortunately, only US and Canadian routes are covered - here's hoping it expands.
On the topic of cheap airfare, be sure to try Farecast. Currently in beta testing, this service goes beyond notifications and predicts future changes in prices.
Another great site (this one good for European residents) is Kayak buzz. If the idea of a cheap trip is more important to you than the date or location, this is what you're looking for. Choose your departing city and see all the offers out there, sortable by price and date.