Dr. Andrew Weil and the Environmental Working Group are two sites that publish updated versions of the dirty dozen produce list. The dirty dozen is a list of fruits and vegetables that should always be purchased organically. The fruits and vegetables on the list are suspected to have the highest level of toxicity when grown conventionally due to high use of pesticides.
Dr. Weil is a holistic doctor who believes diet plays a large role in a person's optimal health. The Environmental Working Group is an organization that works to protect the health of people through activism and research. Both sites also provide information about the clean 15, which are 15 foods consumers do not have to buy organic.