Our favorite airfare search site has just unveiled an exciting new service called Fare Guard.

One of Farecast's features is predicting whether airfare will rise or fall within a certain time span. Now, with Fare Guard, Farecast will lock in the low fares for you.

"Fare Guard allows customers to protect the lowest fare for one week without buying a ticket (when we predict prices are going down). Essentially this product puts our money behind our predictions," David Shim, a Farecast spokesman, tells us.

Having a fare locked costs $1 during the test period and will go up to $9.95 after the full rollout.

Try it out at Farecast (Small registration required). — BEN POPKEN