We're not touting Spirit's $9 Fare Club, but quite honestly, they do have some great deals. You can do a trial membership for $9 (3 months) and a full year is $40. We joined so that we can check to see if seats are available for the fares they list, but we've never taken advantage of them (hey, we're too busy finding fares here.)
Right now they have a sale on a handful of dates for each route that's on sale. For example, Atlanta to Ft. Lauderdale is $16 RT, as is Columbia, SC (a new city for them) to Lauderdale. Check out the entire list. This sale ends at 2 PM ET on March 5, but don't worry, another one is sure to follow in short order.