Environmental Working Group's Shopper's Guide of Pesticides

These have the most amount of pesticides so should be bought organic:
Celery
Peaches
Strawberries
Apples
Blueberries
Nectarines
Bell Peppers
Spinach
Cherries
Kale/Collard
Greens
Potatoes
Grapes (Imported)

This is a list of produce lowest in pesticides:
Onions
Avocado
Sweet Corn
Pineapple
Mangos
Sweet Peas
Asparagus
Kiwi
Cabbage
Eggplant
Cantaloupe
Watermelon
Grapefruit
Sweet Potato
Honeydew
Melon

They have a lot of good information so please visit their website for more information:
http://www.ewg.org/