Use them to store onions or garlic! Put one onion in the toe, tie a knot above it, put another onion, tie another knot, and so on, then tie the top (cut legs off the panty part) to whatever hook or bar you have handy. When you need an onion, cut off the bottom one below the knot. Keeping onions/garlic separated prevents rotting so they last for months. (Mold still can cause problems, so check onions for spots of mold before you put them in the pantyhose, and set aside any moldy ones to use first.)
If your washing machine drains into the sink (instead of having its own drain into the sewer) put pantyhose over the end of the drain hose and knot to hold it in place. It will catch lint and prevent the drain from clogging.