Ask HN: What is your plastic sheet for the web?

I once was listening to a survivalist talk about how, if he had to pick any tool (beyond clothes/boots) to survive in the wild, it would be a plastic sheet/tarp. He had various uses for the sheet: shelter, collecting water, etc. He was certain it would make up for any other choice of tool. So simple, but so much utility. I'm curious, if you were going to open up a computer and head out into the web, what (beyond "clothes/boots") would be your single tool of choice?