LivingSocial is a website that specializes in coupons and deals on the internet and each day, they offer a daily deal, with many of those targeted for the city in which you reside. Often, these are discounts on meals or massages, but today's deal is a $20 Amazon Gift Card for $10. You do have to either create an account or sign in with Facebook and then use a credit card to purchase the gift card (if your card lets you set up a one-use card number, this is a good place to use it). It looks like this will be one of the most popular daily-deal coupons in recent history, even more popular than Groupon's Gap deal, according to Techcrunch.
I read thru the fine print and this appears to be a standard amazon gift card, so can be used on Kindle books, unlike some Amazon promotional codes that exclude them. The deal is only live today, though, so you'll want to take advantage, if you can; the deal is limited to those in the US.
An added note - you don't get the coupon right away. They have a "share" option that can reduce the price to zero (for this particular deal) and in order to take advantage of that, you don't get the gift card until the next day. You can also cancel the purchase on the My Deals page, up until the time your card is charged.