I have a large ziploc bag hanging out in my freezer. I put in carrot tops, tomato tops, green onion tops, citrus peels, garlic tops, celery tops, pepper innards...basically anything that has nutritional, tasty bits that aren't really good for eating/recipes.
This weekend, I boiled the whole collection for 30 minutes or so... until I cooked the color out basically. Then I strained the whole mess, and put the broth in a pitcher.
All ready to use in recipes and all kinds of deliciousness!