I use them as heavy rubbish bags (we're still clearing out old sheds though
) I've also grown potatoes in them, just roll the tops down and unroll and fill with soil as the haulms grow. I'm sure they have more uses though - than my limited ones
I've seen them used as roofs on chickn houses and very basic pig shelters (pallets for sides, chicken wire and plastic bag roof) Failing all that, try your local recycling place
Karen