Beginning June 20, 2007 budget carrier Spirit Airlines will  charge for every piece of checked luggage and they only gurantee that they'll have room to carry one checked bag per customer:

"Spirit Airlines can only guarantee the transportation of one (1) item as checked luggage per fare paying customer. Additional bags can be purchased on a first-served basis; but Spirit cannot guarantee that there will be space for extra bags, and will not be held responsible for any additional items brought to the airport by the customer. Please note that space is extremely limited on certain flights. Please see Excess Baggage for more information."

 Thankfully, they're sticking to standard carryon policies: one bag of 45" or less combined dimensions (40 lb. limit) plus standard personal item.