http://gardening.about.com/od/totallytomatoes/qt/Tomato-Companions.htmPlants Recommended for Companion Planting with Tomatoes: amaranth, asparagus, basil, bean, calendula (pot marigold), carrots, celery, chive, cleome, cosmos, cucumber, garlic, lemon balm, lettuce, marigold, mint, nasturtium, onion, parsley, peas, sage, stinging nettle and sow thistle.
Amaranth helps repel insects.
Basil repels insects and disease, improves growth and flavor. Repels mosquitoes and flies (even fruit flies).Borage improves growth and flavor and repels tomato worms. (I’ve not found this to be true.)
Bee balm, chives, dill, mint and parsley improve health and flavor. Use dill early since mature dill starts to inhibit tomato growth.
Carrots planted near tomatoes may not get as large as they should, but they’ll still taste good.
Garlic repels red spider mites. Garlic sprays help control late blight.
Stinging nettle nearby improves taste.
Sow thistle aids growth.