There are a couple of great new lights from Raleigh, one with an alloy body, the other with a plastic body (£24.99/£19.99). Designed for the commuter more than the offroader, the Powerbeam lights will save your bacon with their one-watt LED beam, should you find your way off road by mistake.
Despite the low power, they are astonishingly bright. There are two settings, flashing and constant, either of which will light you up like a small beacon on your ride home - or to the pub. Four AAA batteries (supplied) keep it running for a fair old while; ours have done about 15 hours so far and are still going.