The Dirty Dozen

According to The Environmental Working Group, these are the fruits and vegetables known to retain the highest levels of pesticides and should be eaten organically grown. They are listed in no particular order.

Peaches
Apples
Bell Peppers
Celery
Nectarines
Strawberries
Cherries
Kale
Lettuce
Grapes (imported)
Carrots
Pears

Conversely they also have a list, The Clean 15, which includes those fruits and vegetables with the lowest pesticide residue:
Onions
Avocado
Sweet Corn
Pineapple
Mango
Asparagus
Sweet Peas
Kiwi
Cabbage
eggplant
Papaya
Watermelon
Broccoli
Tomato
Sweet Potato

So if you have to pick and choose which fruits and veggies to buy organically, make sure you buy those on the dirty dozen list organically grown.