Spirit Airlines is the king of the our-site-only sale fare. You won't find these deals on Orbitz, ITASoftware, Kayak, or Travelocity, and they're great bargains.
The current sale comes in two parts: one is date specific, mostly for six or so specific dates in December and January, with fares starting at $9 each way for Spirit's $9 Fare Club members, and $28 each way for non members. The second is for travel Tue/Wed Dec 9 to June 17, and other days for travel Mon and Thu or Fri, Sat and Sun, travel through Feb 10.
The date specific sale fares must be booked by Dec 2, midnight ET; the others by Dec 3, midnight ET.