If they're fairly clean, grocery bags and produce bags can be used to hold a large brick of cheese - just don't use it for cheese again after, because the second brick will go moldy faster. In Canada we get our milk in bags; the outer bag is great for homemade bread - again, once only. The smaller bags are good to freeze things. I stuff three produce bags into a small jar from single-servings of jam and keep it in my purse, for such things as diaper emergencies or needing a bag for garbage, etc. One time I braided together plastic bags and sewed them with more plastic bag, to make a small rug for my friend's door. :P I don't know how it ended up, but at least the bags weren't just jammed into a crevice in her small dorm.

I imagine a bag could make an interesting improvised watering can - nip off a corner when you already have the full bag of water at the garden.