I recently played an off-island festival where a critical piece of gear was DOA, and I had to play three dates without it. Everything was fine, but it made me think about what we consider essential. I thought I couldn't get by without that piece of gear, but three successful shows proves otherwise. Would it have been nice to have? Sure. Could I get by without it? Sure.
So, in re-assessing what I can't get by without, that's probably going to be my capo. A guitar goes without saying.
What piece of equipment can you not get by without?