Yahoo FareChase Alerts finds the lowest air prices for you and brings them right to your desktop. Just select the flights you want to track using the plus button on the right of Yahoo FareChase Alerts and you'll see the best price found recently on Yahoo FareChase for round-trip travel on that flight.
You can also set an alert for when the flight drops below your max price. Then, click on a price to do a real-time search on Yahoo FareChase. This version is the first release on CNET Download.com.