O2 currently have the Griffin Powerjolt Reserve for sale for only £10 up till the 19th of January. These things usually retail for £30 but they are having a sale on iPhone accessories and they've reduced the price of this down to £10 in error. Price for it should be £20 but due to a POS error they have them for £10.
The Powerjolt reserve works as a in car charger using your own USB charge cable and also comes with a removable clip on battery suitable for use when you're out and about and running low on juice.
These retail for about £25-£30 so I thought £10 sounded an awesome price especially considering it comes with the emergency battery.
To read a bit more about the device you can look here on the Griffin website: http://www.griffintechnology.com/products/powerjolt-reserve