Onions are a safe companion. Consider, if growing herbs, to include basil as a real quality companion. The list following is a part of my companion chart compiled from comparison of many sources. A copy is available on e-mail request as a Word doc.

Tomato GOOD COMPANIONS - Asparagus, Basil, Bush Bean, Carrot, Celery, Chive, Cucumber, Garlic, Marigold, Mint, Nasturtium, Onion, Parsley, Pepper
BAD COMPANIONS - Corn, Dill, Fennel, Pole Bean, Potato