Spirit Airlines will now charge you $100 at the gate for a carry on bag. Pay online, save $65. FOR A CARRY ON.
While I'm bloody sick of everyone dodging the checked bag fee to lug a weeks worth of luggage down the isle and stuff two over head bins... this is probably not the answer.
Ideally I'd prefer they just weigh passenger + baggage and alter the ticket price accordingly. At least then I won't be subsidizing the fatty spilling into my seat and the notion of "checked" vs "carry-on" goes away since everything is a flat price we're back to the "does it fit in this box?" criteria of carry-on.