Other than the essentials every photographer needs for their equipment, there are certain items that everyone should have with them no matter what camera system you use.
1) 6-Port USB Adapter
4) Ear Plugs
I am not talking about ear plugs for listening to music, but ear plugs for protecting your ears in loud environments.
5) Paint Brush
You may be asking yourself, why would you need a paint brush with your camera gear? When I shoot in a dusty environments, I can easily brush off any dust or dirt off my gear where an air blower won't do the trick. If you're shooting products, you can also brush the dust off the products. Its a great accessory to have to clean off your gear. I would recommend getting a medium sized brush that is very soft.