Okay now I just read about this not too long ago on the Canadian trips forum where they keep talking about DING and the unreal cheap fares they are getting. It is a US program, I had to ask what it was because I've never seen fares this cheap--maybe someone here can give you more details--this is what I found out when I searched on the forum:
"It's a program that you can download onto your computer. It's a Southwest airline savings program. Most days, sometimes even twice a day, they send out fantastic savings on flights. You only have a limited time to book (hours...sometimes minutes if you get in there late!!)."
"DING! is a program offered by Southwest airline, very low and cheap fairs are offered to you when you get "Ding-ed", that is, if you're subscribed to their program.
Go to southwest.com and download their Ding program"
"DING is a downloadable programme from Southwest Airlines It sits on your desktop, and every now and then (twice a day, sometimes), you get DINGed, with a list of DING specials. They are a timed special-so you have to respond asap, and within the time limits. You'll need a zip code when you download the programme. It doesn't cost anything, doesn't add any junk, and is a sweet way to get a bargain. Oh....and the ONLY way to get a DING fare, is to use the link in the DING 'email' itself."
Hope this helps!