Some plants make good neighbors, some don't. Without much scientific corroboration, gardeners throughout the centuries have practiced the folk art of companion planting. Plants out in the garden have very specific likes and dislikes. Give it a try! If it doesn't work, you haven't hurt their feelings!
Two very nice little reference books by Louise Riotte are:
If you'd like to give it a try, here's a short list to get you started.
Anise Close to coriander, far from carrots.
Asparagus Thrives near basil, parsley and tomatoes. Hates onions.
Basil Great companion for tomatoes and asparagus.
Beans Great companion for tomatoes and asparagus.
Cabbage The whole family (brassica) love herbs and get along with beets, cucumbers, lettuce, onions, radishes potatoes and marigolds.
Carrot Plant near tomatoes, cucumbers, beans onions, lettuce, and peas. Keep them away from dill.
Cucumber Grows well near radishes, beans, members of the cabbage family, tomatoes, peas, marigolds and oregano. Keep away from sage.
Dill Grow near onions and members of the cabbage family. Keep away from carrots and tomatoes.
Eggplant Grows well near beans, peppers, tarragon, thyme and marigolds.
Garlic Repels many different garden pests. Grow anywhere sunny.
Lettuce Plant near cabbage, beets, carrots, chives, garlic and onion. Doesn't like broccoli.
Mint Attracts many beneficial insects.
Mustard Likes beans, hates turnips.
Nasturtium Traps aphids, repels many pests. Good companion for cabbage crops, beans and peppers.
Onions Set near beets, carrots, lettuce, spinach, potatoes, and cabbage crops. Keep away from sage and peas.
Parsley Good near asparagus, corn and tomatoes. Repels pests.
Peas Likes tomatoes, eggplant, lettuce, spinach, peppers and corn, radishes, cucumbers, carrots, and beans. Keep them far from garlic and onions.
Peppers Plant near carrots, onions, marigolds, basil, marjoram or oregano. Avoid beans, eggplant, tomatoes, fennel and kohlrabi.
Radishes Interplant with lettuce, cabbage, tomatoes, beans or beets. Good with peas, parsnips, onions, carrots, cucumbers or squash.
Spinach Good companion for eggplant, cabbage crops, peas, onions, marigolds, beans. Not so friendly to potatoes.
Squash Plant with beans, corn, sunflowers, marigolds, radishes, mint. Keep away from potatoes.
Tomatoes Plant near asparagus, cabbage, carrots, cucumbers, onions, peppers, marigolds, basil, parsley, and sage. Keep away from corn, dill, fennel, kohlrabi and potatoes