DING! is a program you download from the Southwest Airlines website. You put the airports you are watching for specials and it alerts (dings!) you as soon as they release a special fare for those locations - you have to book ASAP through the SWA Ding site to get those great sales though.
Here's a little info from SWA re their Ding program and there's much more about it in their Ding! FAQs section of their website:
Why should I download DING!?
DING! delivers exclusive offers that arent available anywhere else. To take advantage of these offers you must access them through the DING! application. We anticipate these offers to be so exciting that everyone will want DING! so that they too can take advantage of the offers.
How frequently can I expect to receive offers?
The application will check for new offers frequently. Watch your DING! application for the latest offers they can come any time of the day or night!
I was away from my desk when an offer came in. Its still visible in the DING! application but it has expired. Can I still get this offer?
DING! offers are only available for purchase through the DING! application and only during the dates and times indicated. If you miss an offer, stay tuned, there will be more offers coming.
How will I know when there are new offers?
The DING! application will alert you with an audible DING! and place an envelope over the DING! icon.
Can I designate more than one home airport or zip code?
You may select up to ten airports in your "My Airports" section. However, you may only enter one zip code, which would represent your home address zip code.
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There's also lots more info (including what fares other people have gotten with DING!) over on the "Transportation" board here at the DIS - click on the link:
http://www.disboards.com/forumdisplay.php?f=16
