Frontier is having a sale, and considering that these are nonstop flights, the fares are quite good.
Some Mexican and Central American destinations are also included, such as Denver to Mazatlan for $99 each way plus tax and Denver/Cabo $129 each way.
There's a 10 day advance purchase required, and for domestic destinations you'll find the lowest fares for travel on Tue/Wed/Sat or Mon-Thu and Sat for international.
Blackout dates Feb 15 and 18. Purchase by Dec 29 and complete all travel by Mar 5.