We always travel with a couple of the really cheap £1 / or $1 plastic poncho's.
If you are backpacking they are often big enough to cover yourself and to keep your back pack dry. You can also use a spare one to cover a trolley style bag.
At the beach put one under you towel or sarong to avoid problems from bugs or damp sand. You can do the same to create a picnic rug on damp grass. Move this up onto a table and you have a table cloth with plastic backing.
Travelling in an open air vehicle in the rain is also more pleasant with your poncho pulled over your legs to keep you dry.
If you are staying somewhere and the mattress is very dubious you can use the poncho as a plastic under sheet.
Cheap light and well worth having around in our opinion and I am sure you can think of many other uses.