A portable battery pack is only as useful as long as you're able to keep it charged. So Exogear has created a new compact power system that uses stackable modular battery packs to prolong its life almost indefinitely—as long as you're willing to carry enough of them as backups.
The $90 Exovolt Plus base unit contains a 5,200mA lithium polymer battery that provides enough juice to recharge even an iPad from a built-in USB port. But if you don't think you'll have an opportunity to recharge the base unit before it's depleted, you can simply connect it to a $50 sub-unit which contains the same 5,200mA battery—minus the USB port. In theory you can stack an unlimited number of the units together for perpetual power, but you're probably going to be limited to how many you can physically carry.