iTechDeals offers the PowerBank Credit Card-Sized 2,500mAh Portable External Battery Charger for $10.97. Coupon code "DEALN2OFF" cuts it to $8.97. With free shipping, that's $21 off list price and tied with last month's mention as the lowest total price we could find. It's compatible with most microUSB-charged devices and fits in a wallet. Deal ends December 20.
Note: It won't arrive in time for Christmas.