Thought I'd share the results of my enquiry:
Allegiant Air
Frontier Airlines
Southwest Airlines
Spirit Airlines
Other useful comments:
--Frontier is opening a hub in Pittsburgh
--Be sure to check bagage policies with Allegiant Air
--It may be worth driving to Youngstown with Allegiant Air, because the parking
is so cheap there
--The closest Frontier hub right now is Cleveland; with more destinations than Pittsburgh, so it may be worth the drive to save money
--Southwest includes 2 free check bags in their fares
--On any airline, the best way to find the cheapest rates to a specific destination is to do a flexible dates search - this is really easy on southwest.
--To search for flexible dates on Southwest, after you put in your cities and preferred dates on the southwest website it will generate a list of options and fares - look at the top of that list and there's a blue box that says flexible date search - click on that and a calendar of days with cheapest fare option each day appears.
--I have friends who have flown on Spirit airlines in and out of the Latrobe Airport. I think that's mostly to Florida - I have not done it myself but they have gotten great deals that way.
Thanks for all your help! Ya'll are the best :-)