If you think you're going to be changing the dates of your flights, your best bet is Southwest (since you're asking about U.S. carriers...).

If you need a legacy carrier, I know AA sells a no-change-fee add on to most fares. They bundle it with a lot of the basic elite benefits, so if you're already elite it's not a particularly good value. (If you *aren't* elite, then it depends on how much you value the elite benefits.) But it's definitely better than buying a fully-refundable ticket, as long as you're fairly certain you *will* eventually fly.