You'll all know that continental brands like Batavus and Gazelle have rear rack lights.
Now you can fit one too at a reasonable cost.
http://www.thebikechain.co.uk/store/product/27954/RSP-CityBright-1-2-Watt-Rack-Light/
There are other brands of course. The fact it's a refelctor and light in one go is nice idea. Not unique but affordable upgrade.
Some makes of rack (like Blackburn EX-1 OR EX-2 don't have a fitting plate so bike bodging skills may be required.
Anyone got any other favourites? Factors should also be how easy it is to change re-chargeable batteries. Dynamo/dynahub ones are good but not common here.