Spirit: DCA-FLL $92 all in (+ $40 membership)
I'm a little reluctant to mention these guys, but the fare is certainly cheap. You have to join their "$9 fare club" (good for a year) to buy these seats, and then they're $92 roundtrip all in. Some availability in June and July, and wide open for several months thereafter.
Obviously a better deal if you're going multiple times or with a group (one membership covers you and anyone you're flying with). Roundtrip purchase seems to be required.
Given how high the prices are on "real airlines" this summer, this could be a plausible alternative. Keep in mind there's a potential pilot's strike this month, though, and that beginning Aug. 1 there's a carry-on bag fee for larger items.
They do have a (complicated?) ff program, but I haven't yet invested the time to see if there are any worthwhile award possibilities.
Spirit also offers other cheapo city pairs through their fare club, but this one seems a bit unique in its easy availability and potentially broad appeal.