Don't you hate it when you get a $100 American Express gift card? No? well, actually, we like that, too. But what we DO hate is when we've used all but $16.43 on it and can't figure out what to do with that balance. And the credit card companies love this, because they legally get to sit on your money and (in some states) perhaps even charge you a fee to "manage" your balance for you (which really is just their way of slowly eroding your balance down to nothing). You don't want to let that happen but... what are you to do?
Buy an Amazon Gift Card with it.
Amazon lets you buy electronic gift cards for any amount and you can send them to anyone, even yourself! Just enter the amount of your balance, make sure you pay with your existing gift card (of course!), and then wait for the email to arrive. When it does, you can then apply it to your Amazon account and — here's the best part — use it as partial payment towards something else. Yes, you're trading one gift card for another, but if you buy anything at Amazon, you'll likely burn this up in a heartbeat.