Ginger

Buying an extremely small piece of ginger from a supermarket, enough for a single recipe, for about 3p or so makes me very happy for some reason. I think because it's one of the few things you have such freedom to buy in any quantity you like from a big supermarket. Still, sometimes you end up buying more and you need somewhere to use all that pungent root.

Make some obviously very gingery things at home, like:
Ginger beer
Candied ginger
Ginger cake

Or you could make more unusual, but still very gingery things:
Ginger and spice ice cream
more to come

Duck fat

Roast a duck and you get fat, lots of tasty fat. There are plenty of very well established ways of using up all that fat, some sort of confit and making roast potatoes is something everyone will tell you. But duck fat is a lot more versatile than just that, it can be used in the place of just about any other fat, for frying, roasting, or even deep frying anything if you have enough. In fact there are loads of awesome sounding recipes for duck fat it some very surprising places.

Duck fat focaccia
Duck fat caramel sauce
Duck fat vinaigrette
Duck fat pastry
Duck fat dumplings

Surprised no one seems to be making duck fat croissants though, seems like an obvious one.

Potato crisps (chips)

It's not usually hard to use up leftover crisps, eating them is usually the best solution. But there are lots of other interesting uses for potato crisps (or chips if you're in the US) that are probably worth buying an overly large bag for.

Salty and sweet always go together so there are plenty of cookie, fudge and brownie recipes that throw in a bunch of crisps (including the brilliant Momofuku compost cookies):

• Momofuku compost cookies (other crisp cookie recipes)
• Caramel potato chip fudge
• Potato chip brownies

Also anything that would benefit from a bit of salty crispiness is a good destination for crisps.

• Meat Loaf's tuna fish casserole surprise
• Crisp coated fried chicken

Then some things just don't follow any sort of reason, like Ferran Adria's Potato Chip Omelete (or Hugh Fearnley-Whittingstall's verison)