Update: Looks like they changed the end date, this one is now expired
This one is an easy one to do, but you do have to have a Android Smartphone, Tablet, or Fire Tablet.
Get a Free $5 Amazon Credit! To get started download the Amazon Underground for Android. Make sure to follow the instructions listed here.
After you do that click here and click on any app. Click “get app” and you are done. Within a few minutes you will get an email stating you got a free $5 promotional credit.
Offer valid for US residents only until December 24 at 11:59PM PST. Credit can be used on items sold by Amazon. Free credit expires at the end of Feb 2016.