I've used the orange Glad "yard waste" bags for many of the purposes suggested.

Every pocket kit has one, because a waterproof layer is one of the hardest things to come by in the bush.

I've cut two of them open to make a temporary groundsheet for my tent (keeps the crud and moisture off, so it's easier to pack up in the morning).

I keep several in my car kits. I always thought I could improvise a big orange "road flare" -- inflate bag, put a LED flashlight inside, and tether to the spare tire -- to keep people from smashing into me on the highway.