Oh hey! I googled your question and found this site with a "companion plant" chart:
http://www.gardentoad.com/companionplants.html
"Companion plants are used to confuse or repel plant pests, to encourage the growth of other plants and to act as a trap for pests and parasites. "Trap" crops draw harmful insects away from the plants you are trying to grow.
Companion plants may also be used a a "nurse" crop--to provide food or possibly an attractive home or habitat for beneficial insects.
Companion plants may produce odors that confuse and deter pests, or their scent may mask or hide a crop from pests."
There is a pyrethrum plant that is said to repel spider mites!!!! (I thought pyrethrum was just a chemical)