This effect happens quite naturally in the wild but must be cultivated in the home garden. The trick to learning what thrives with what and what won't takes many years, with resulting failures and successes. Luckily, other gardeners have been there done that and were willing to share the results of their learning experiences with the rest of us.
By planting certain flowers as companions to your veggies you not only add color and attract beneficial bees but provide your garden with their own personal chemical free pest control agents.
Edging your veggie beds with marigolds, tansies, four o'clock, clover, nasturtiums, petunias, delphiniums and geraniums not only add color to the edible garden but are also wonderful for pest control. 
Asparagus and collard greens like to hang out with marigolds and nasturtiums.
Beans, basil, eggplants, pumpkins, potatoes and tomatoes benefit by being planted alongside marigolds, but remember to not plant pumpkins, potatoes and tomatoes near each other as they can contaminate each other with blight disease.
Clover planted along with cabbage repels cabbage aphids and cabbageworms by increasing ground beetles that take care of the pests.
Tomatoes, potatoes, peppers, eggplants, carrots and brassicas benefit from being planted close to any plants in the alliums family. Alliums include onions, chives, garlic, garlic chives, leeks and others. The blooms of alliums are edible.
Peppers benefit when planted near geraniums and petunias.
Brassicas (all plants in the cabbage family; cabbage, Brussels sprouts, kohlrabi, cauliflower, etc) benefit from being planted near geraniums and petunias.
Other edibles that do well planted among or near geraniums are tomatoes, peppers and grapes.
Lupines are good companions for cucumbers and other gourd type plants and brassicas, such as cabbages, kohlrabi, Brussels sprouts and cauliflower, lettuces, strawberries, dill and rosemary. Lupine in turn benefits from being planted near summer savory.
Tansy helps cucumbers, squash, raspberries, black berries, roses and corn by repelling sugar ants, several predatory beetles and mice.
Zucchini and cucumbers love to be near nasturtiums. As we learned on Part 2 of the 3 previous Companion Planting articles, nasturtiums improve the growth and flavor of cucumbers.
Celery thrives when planted near cosmos, daisies and snapdragons.
Corn enjoys the company of sunflowers, white geranium, nasturtiums and morning glories.
Also, cucumbers, corn and tomatoes do well near sunflowers but potatoes and pole beans don't!
It is important to remember that just because certain flowers are beneficial as companions to certain garden veggies does not necessarily means that all of the flowers will also be edible.
For more information on what flowers are edible and which to avoid, please click on the following Edible Flowers Chart link:
http://homecooking.about.com/library/weekly/blflowers.htm
To read the previous posts on this same topic, please click on the following links:
The Home Farming Project - Edible Landscapes
The Home Farming Project - Victory Gardens
The Home Farming Project - The Recycled Garden
The Home Farming Project - Lasagna Gardening
The Home Farming Project - Growing your own small garden bounty
The Home Farming Project - Share the Harvest
The Home Farming Project - Growing Herbs in Your Garden
The Home Farming Project - Straw Bale Gardening
The Home Farming Project - More on Organic Gardening
The Home Farming Project - 'Potager' Gardens
The Home Farming Project - How to keep birds from eating your seeds!!
The Home Farming Project - Easy Free Mulch
The Home Farming Project - Composting with more than grass and peelings...
