The Astro Anker Mini is on sale over at Newegg for $10.99 with the coupon EMCWWWW33 until 11/13.
This 3000mAh battery is small, portable, and in a durable metal case.
There are higher capacity batteries, but the combination of form factor makes this a good choice for emergencies.
I keep two of these in strategic places for emergencies.
