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Back to the Basics.
Basically, it's the practice of growing specific plants together to deter pests and/or enhance the flavor of the plant.
Tomatoes
Asparagus, basil, bean, cucumber, garlic, head lettuce, marigold, nasturtium, onion, peas, peppers
Peppers
Cucumbers, eggplant, tomato, Swiss chard and squash. Herbs to plant near them include: basil, oregano and rosemary.
Beans, corn, cucumbers, icicle radishes, melon, onions, pumpkin, Borage, Dill, Marigolds, Nasturtium, Oregano
Carrots, celery, chards, corn, eggplant, peas, brassicas,
beets, radish, strawberry and cucumbers
Beans, Corn, peas, beets, radishes, carrots,
Dill and Nasturtiums
Beans, peas, peppers, spinach, tarragon, thyme and marigold
Radish
Radish, beet, bush beans, pole beans, carrots, cucumber,
lettuce, melons, nasturtium, parsnip, peas, spinach
and members of the squash family.
Bean, cabbage family, tomato, onion and roses
Corn
Beans, cucumber, melons, morning glory, parsley, peanuts, peas, potato, pumpkin, soybeans, squash and sunflower
Planting
chamomile with onions improves their flavor. Other companions are
carrot, leek, beets, strawberries, brassicas, dill, lettuce and
tomatoes. Intercropping onions and leeks with your carrots confuses the carrot and onion flies