I first read about this idea in the The Locavore's Handbook. Instead of going into the compost pile, you can save your vegetable trimmings in a bag in the freezer. When you fill it, simmer the contents on the stove for a half hour. You then have stock for soups or risotto. Its a great way to utilize all of those small center pieces or garlic (no peeling needed), celery greens, or parmesean rinds.