Check here for my report . A court has found in favor of a traveler who was being threatened by Delta. Use at your own risk. Northwest's Cyber Saver Fares. They require a 3 day purchase window and you must fly the first Saturday and return the following Monday or Tuesday.
International fares give a wider travel window.
I don't think they're that good if you just look around a bit. Here is my review. Southwest frequent flyer, weekly specials and E-Bay SW has a very different program. If you know the rules, you can earn a frequent flyer ticket after 4 round trips within a 12 month period. Southwest's weekly specials are different.