Frontier Airlines is running a tax day special offering a 15% discount on domestic fares. You can take advantage of this offer by booking flights between now and 11:59 PM EST on Thursday April 17th for travel anytime through June 12th, 2014. Friday and Sunday travel are blacked out. This promotion is stack-able with other sale fares. Make sure you use promo code REFUND when booking.
Keep in mind that Frontier is an ultra-low cost carrier comparable to Spirit and Allegiant meaning you will be charged for any little extra item including a pop or snack. However, unlike Allegiant, you will not have to pay for carry on bags as long as you book via Frontiers own website (which you will have to for this offer anyway).