You know some people who carry "power banks" which are external battery packs for use when their cell phones are low on juice? This is the Fuel, the smallest in the family of such accessories. Purchased from Kickstarter, this pair was just delivered today, missing Christmas by a pinch of hairs.
While most other power banks sport fancy schmancy designs, Fuel has gone with a very utilitarian jerry-can look. Given the choice of red or green, the answer was obviously both.
Size-wise it's clearly larger than a 50-cent coin, but it's quite light and should still fit comfortable in a jeans pocket.
While it comes with a key-ring loop, it doesn't come with its own micro-USB cable, but since you're already charging your phone with one you don't really need the extra clutter of another one.
With the little rubber cap off you plug your phone in at the top. iPhone users will, of course, order a Lightning-compatible connector instead of the standard micro-USB one. Below, there are three LEDs that indicate charge status. There is also a nifty on/off switch to prevent the unit from discharging power when not in use.
A couple of small practical issues, though: you have to remove your phone cover to plug it in properly as the connector isn't long enough to accommodate the extra layer of silicone or plastic. Secondly, it fits into your phone cross-wise, so you don't want to have both devices connected to each other when you put them back in your pocket, unless you like your pockets lumpy for whatever reason.
Fuel is no substitute for your regular phone battery, but it is a handy emergency stand-by for a few minutes extra talk-time or sending a desperate slew of text messages out until you are able to charge your phone again.