What is one piece of gear that you cannot live without?

Hi fellow redditors, I've purchased my camera a while back with 2 lenses. I am now looking if I need or want something else. So excluding the obvious (body/lenses/SD's), what is a piece of gear in your bag that you simply leave home for a shoot? Something that changes your workflow or process?

For me, it is the Peak Design's Capture clip's for attaching my camera to my backpack so I can easily have my hands free for showing poses or anything like that.