Mr. ETB thinks this is nutty, but I save the paper wrappers from sticks of butter. There's always enough left behind on the paper to grease a bowl or a pan. They don't take up much room folded in the fridge and even keep your hands nice and clean when you grease a pan.
Some people even save the wrappers and cut them into squares to wrap caramels or taffy. That's kind of a cute idea.
Then, there's the boxes:
I keep a file filled with collapsed cardboard boxes from things like cereal and crackers. You never know when you'll need to build a robot (oatmeal cannisters work well too), make a calendar, or have a sturdy surface for gluing construction paper collages to. If you take them apart, they stay flat and you'll always have an assortment of sizes on hand saving all that cutting.
Plastic fruit containers:
If you're lucky enough to afford berries in those large plastic containers, wash them out and save them. With layers of waxed paper between, you can use them to ship cookies and candy to the people on your gift lists. Because they have holes, you need to give the boxes a final wrap in cling film, but that's a good idea anyway.
Save run stockings to use for propping up plants in the garden this summer. One pair will hold up three or four tomato plants once cut into lengths.
Soda Bottles/Juice Bottles
As I get er...older, I find it easier to pour than to scoop. When I purchase large bags of sugar or rice, I pour them (with the help of a funnel) into plastic bottles. I keep the extra on the pantry shelf and the one I'm using on the counter. This is so much easier than tripping over large food storage buckets (and cheaper). It does not work well for flour, but things that pour like beans and grains are kept in neat order.
OK, let's go save some money in the new year.