i have various battery lights for my commuter (the new Cateye LED is awesome!), mountain and road-race bikes, but for my city-bike i have a good old European dynamo --- ALWAYS there and you never forget it at home and the batteries never go dead.

dynamos are perfect for riding in the city and probably 70% of the German cyclists use dynamos - most are the tirewall type but there are others. in the US they are very hard to find and most bike shops either look at you funny or laugh at you if you ask for a dynamo - "man, that's like OLD technology. join the current century dude..." they're usually not bright enough (2-5 Watts) to light up totally dark paths, but to BE SEEN they are ideal.