Allegiant Airlines is having an advertised sale on many routes, but we're finding that there even better unadvertised fares, as is often the case with those wacky airlines.
As you can see from the fare search on the Allentown to Orlando route below, there are lots of dates as low as $29 each way and some, such as Chattanooga to Florida, at $19 each way. Another example: Eugene to Las Vegas also $29 each way, all on nonstop flights.
Similarly low fares on other routes. To find low fares on Allegiant, go to their home page and choose the route using the drop down menus, and click the "flight only" box. Enter in approximate dates and you'll automatically see a flexible date search calendar, similar to the one below. Use the forward arrows to see other months of travel. Allegiant is currently selling seats through the end of July.