Reelight is a Danish brand new to the UK and the SL100 is a simple but very clever light system.
A white front and red rear flashing unit with large (1½in) reflective lenses mount via simple stainless steel brackets to the hub axles on the front and rear of the bike, then two large magnets are fitted to the spokes so they pass the centre of the light's head unit; as the wheel spins, each pass of the magnet creates enough power to make the unit flash. Simple but brilliant. Fitting is an absolute doddle and although the instructions say the magnet must pass within 3mm of the light unit we found this somewhat over cautious; the light still operated at more than twice that distance.
The units are very robust and have stood up to adverse weather extremely well. Obviously these aren't lights to see by, but they do a great job of helping you be seen and they comply with the UK's legal standards for flashing lights. As a back-up system I'd recommend these wholeheartedly as they never need batteries and are fixed permanently to your bike, without the drag or increase in tyre wear found on traditional dynamos, or any delicate wires or cables to worry about.
The only downsides we've found are that, on this the basic system, when you stop, they stop, and also, the axle mounted position is quite low although if a car is close enough that they can't see the light they are already too close to you.
A version with battery back-up is also available, which will flash for two minutes after stopping. The version tested here is for fixed/single or hub geared bikes. A derailleur version is also available which has its rear bracket shaped to fit around the cassette.
In all we would recommend the Reelight wholeheartedly for your winter/commuter or training bike as you'll never forget your lights or run out of battery power again.