The KenBuster is a new device which helps you avoid being fined for driving in to London's congestion charging zone when you have forgotten to prepay the charge. It uses a mixture of GPS and GPRS to track where you are and notify you when you are approaching a congestion charge zone so that you do not get fined. It is about the size of a mobile phone and when entering a congestion zone it automatically triggers a payment to Transport for London if needed. So that you do not pay when required (off peak hours) it also knows the time and will only trigger a payment when it is due.
This is not a bad idea for those who drive regularly in London. For more details on the KenBuster check out KenBuster.