The ChargeKey is a little USB key that you can clip on to your keyring to use as an iPhone charger when you don't have your Apple-specific Lightning USB charging cable with you. There'a also an Android Micro USB equivalent. You still need a USB socket to plug the ChargeKey into, of course, but if you have one of those to hand then you can easily charge your phone back up to capacity.
A month down the road and I've used it occasionally, usually while sitting at my desk at work or when I've been at friends' places. It's been convenient to have the Nomad ChargeKey around, and I'd recommend it, but as someone who likes to take lots of photos with my iPhone and track my walks and runs with battery-intensive apps, I think the more important thing for me will be an external battery that I can keep in my bag to recharge whenever necessary, when I'm away from the computer.
The ChargeKey is available on Amazon in two flavours, one for Android and one for iPhone at a price of £16. Nomad also make creditcard shaped chargers, known as the ChargeCard, but my wallet is already stuffed to bursting point with old receipts and loyalty cards, so I didn't try that one out.