Well, some of it depends on what kind of diving you do.
I think most diving benefits from having a good, small light. Even in the tropics, it's nice to be able to look into holes and under ledges. In murky water, a bright, well-focused light is a communications (and to me, a safety) tool.
If you do a lot of diving off of boats, I think you will want signaling devices, because being separated from the boat is real possibility and it's pretty stressful, if you can't get their attention. Signaling devices can include air horns (Dive Alert), whistles, mirrors, and safety sausages. A lot of us like to use a sausage-like device that can actually be deployed while you are underwater (Surface marker buoy) so that the boat knows where you are before you even get to the surface.
Communication between buddies is mostly done with hand signals, but if you have an issue too complex to get across that way, it's nice to have something to write on: A slate or wetnotes.
I don't consider it an accessory (to me it's an essential part of my kit) but a compass is very handy to stay oriented and to get back to your starting point.
If you mostly shore dive where there is surf, a finkeeper clip to secure your fins to your BC while you cross the surf line can be very useful.
Hope that helps.