Lights and something to carry your stuff in.

I use a mirror, am getting some fenders put on, have a cargo rack, etc. But at the end of the day, I have lights on my bike and a messenger bag that holds all my junk. Water's nice too.

That being said, I do a stretch on an overpass with no shoulder and heavy traffic followed by a left turn into a parking lot. The cheap handlebar mirror I picked up a few weeks ago has been a major quality of life improvement.