Amazon offers a $1 Amazon MP3 Credit for free with purchase of select Android apps. That continues to be the best Amazon credit freebie we've seen this year, particularly since many of the qualifying apps are free themselves. You'll receive an email after purchase that confirms the credit applies to your account automatically. Deal ends December 31.

Note that it can only be claimed once per Amazon account, so if you've already opted in this year it won't apply again.