Eating food that grows naturally during the season and is grown locally not only saves the earth by not requiring more energy to grow and transport, but also tastes a lot better. I bought some juicy ripe cherries and strawberries yesterday at Rainbow Grocery. There is nothing like eating fruit that was picked ripe.
Here are several guides to summer's produce:
CUESA: Vegetable Calendar (there are links to the season's fruit, nuts, flowers, plants, meats and cheeses as well)
Food Network: Summer Fruit & Veggies
Epicurious: Seasonal Ingredient Map (find seasonal produce by month and by state)
KQED: What's in Season? (seasonal guide that shows newly in season, peak season and winding down)