Don't know if anyone has posted this yet.
I keep a one of those cheap disposable big box store painters tarps in my pack. It weighs just a few ounces and takes up about as much room as a smushed hot dog roll. I've stuck it to a leaky tarp with neodymium magnets, made atarp extention, used it as an extended porch or vestibule, poncho, ground cloth on wet ground, sit pad on wet surfaces. The only problem is they are thin and not very durable, but very handy to have one on hand!!! Wouldn't go out without one!!!