Lots of limitations (and fees) to be aware of ... but at $15 (total including fees!) per one-way flight, it might be worth reading on ...
Check out Shawn's post over at Miles to Memories HERE.
$15 one-way airfare is great. Lots of cities available to/from certain cities, but only Denver (Frontier's hub city) from PDX. But you could book a $15 flight to/from Denver, and then another $15 flight to a LOT of other cities.
This deal is only available TODAY, using a code "SAVEBIG". The biggest restriction is that you can only fly on Tuesdays and Wednesdays at this fare, now through November. And this is a "while supplies last" deal (supposedly they are selling up to 100,000 tickets at this price).
Also be aware that Frontier (like Spirit Airlines) is a "low fare" airline ... which is code for "we charge fees for everything!" Most notably, they charge fees for not only checked baggage, but also carry-on bags (you have a free "personal item" up to a certain size).