As demonstrated by Glitter and Guttertrash, you can turn your green onions purchased from the store into an onion-producing plant. Use the best parts of the onion in your cooking and save the bottoms (approximately 3 cm of stalk with the roots attached). Place the cutting root down into some potting mix or your garden bed with the stems poking out of the surface. A few weeks later, they'll look as though you just purchased them from the store. This can be done time and time again, saving money and time watering.
Other vegetables that can be started in a similar fashion, with a little luck, are garlic, potatoes, pineapple, whole onions, celery and parsley.
(Image from Glitter and Guttertrash)