iOS: Cookmate helps you come up with meal ideas based on what you've got in your pantry, and when you need one or two extra ingredients, it creates a shopping list you can pull out at the store.
Your fridges, pantries and cupboards are usually filled with an eclectic mix of ingredients you bought for other meals, but for most of us, it's not easy to think of recipes looking at any old combination of ingredients you have on hand. Cookmate allows you to select the food that you have got on hand, then generates a list of recipes for you to choose from. Once you got the recipe that sounds most appealing, the app tells you what ingredients you're missing and adds them to your grocery list. So you may still need to buy groceries, but the important part is that you're still using what you've got (and spending less at the end of the day), making it a great tool to help reduce the amount of food that you waste.
Cookmate is a free app available through iTunes. You can add extra recipes by purchasing them through the app.