Here's a link to good companions for tomatoes with info about what the benefits are: https://gardening.about.com/od/totallytomatoes/qt/Tomato-Companions.htm
Ones that I've used most are carrots (I have a book on companion planting called Tomatoes Love Carrots), marigolds, nasturtiums, basil, garlic. Some people say you should grow your spaghetti sauce all in one spot: tomatoes, oregano, basil, garlic. However, the oregano tends to take over whatever bed it's in, so I don't do that.
Things not to grow with tomatoes include cabbage/broccoli, corn, dill/fennel, eggplant, potatoes, peppers.