Lights

i wouldn't use a knog or similar as my sole souce of being seen on the road. they are just not bright enough.
one of these

or one of those fenix (sp?) torches

I had one of these, but it got nicked. One of the disadvantages if you forget about em :)
Also, I found that when you put it in your bag it tends to switch on very easy and when you get it out, its flat. Bad design IMO, won't buy again for that reason, everything else about it is great..