Southwest Airlines is well known for their great low fares which do not add on extra fees for baggage, and they will email you (as will all airlines) their latest sales promotions. Wouldn't it be nice if instead of scrolling through all the offers, you were instantly notified when offers for discount fares to your preferred destinations showed up on your desktop?
Ding! is the newest Southwest innovation; it's an application that you download onto your desktop. You select the destinations you travel to, or want to be notified of discounts for, and you will literally hear an audible "Ding!" with an icon showing you when there is a discounted fare available. The Southwest system notifies travel agents when fares change instantly on their computer reservation systems; this way you get the changes too, as immediately as the agents do, but without requiring that you check your email or call the airline.
Making your travel life easier while saving you money on your trip is the mark of great service; having your computer do it automatically, with no cost of time or money, is the best service.