With the Colony Collapse Disorder affecting the presence of bees, it's a good idea to plant flowers that attract pollinators, to improve the success of crops and increase the local bee and butterfly population.
Common flowers that are attractive to pollinators are poppy, lupine, viola, wallflower, black-eyed Susan, Butterfly flower, Calendula, Dahlia, Amaranth, Marigold, Salvia, Sunflower, Zinnia, Portulaca, Lavender, Foxglove, Cosmos, Coneflower and Echinacea.
Anybody who has run barefoot in a yard knows that clover attracts bees. It is a great ground cover plant too.
Garden plants that are highly attractive to bees and butterflies include parsley, cilantro, thyme, dill, basil, chives, peas, squash and pumpkin. Here's a link about planting to attract beneficial insects: www.reneesgarden.com/hm-gardnr/resource/bee-list.htm