Wow, this is super, and no word on how long it's valid for, but use promo code 50%off in the promo code box at Allegiantair.com and you will get 50% off your flight. Just be sure avoid extra fees and "upsells"; buy your ticket at the airport and not online to avoid the "convenience" fee; don't check bags. We found seats on the Allentown/Mrytle Beach route for 1/2 off, but the code doesn't work on all routes or dates.
Airfarewatchdog, by the by, is the only airfare alert service that includes Allegiant. Their fares do not appear on Kayak, Orbitz, etc., or via ITASoftware.com.