Spirit Airlines has one particular fee which is entirely absurd. In addition to the requisite checked bags fee and carry-on bag fee, you're charged a fee for booking a flight over the phone...or online. Yes, the booking option which involves no actual person, and so is cheapest for the airline, costs money. The only way to avoid the fee is to book your ticket from a live agent at the airport.

Clearly, they do it because booking a flight at the airport is absurdly inconvenient and so few people will do it, rather than as a means of defraying the cost of hiring a customer service agent. But that, combined with their nasty reputation of seeking to extract every last dime from your wallet (someone else mentioned Ryanair as being as if Ayn Rand started an airline company, and Spirit certainly seems like that as well) prevents me from being willing to ever book a flight with them. Even though, counting the fees, they've often been the cheapest option on a flight I've needed, I prefer the second-cheapest airline, which is usually Southwest (unless I don't need to check a bag, in which case United usually wins).

I came here to say exactly that. Southwest feels reasonably mainstream to me these days. (And Spirit is terrible.)

Frontier as well, I learned recently.