Farecast, w/c bills itself as the first airfare prediction site, will be guaranteeing that each ticket price listed on their site will hold for a week; if the price rises, they'll pay the difference. The service is called Fare Guard.
Sounds good, right? Most travel agents will lock a ticket price for 24 hours, but Farecast does that for a week. You will have to pay $10 for the service, w/c is discounted to an an introductory price of $3 till the end of the month. Also, they don't offer it on every search.
How It Works
You go to their site and enter the usual info (Leaving From, Going To, When, Number of Adults). The results show what the available fares are for different airlines.
For example, you see a fare at $250, which Farecast predicts will be the lowest price available for the date you've entered. If they're offering Fare Guard for it, you can buy the insurance for $10.
Book within the next seven days and if the price at the airline happens to have increased to $300, Fare Guard will reimburse you the $50.