How do you know which weeds you are dealing with? Check out Rutgers Cooperative Extension Weed Gallery, which lists the most common weeds in the Northeast. Their Thumbnail Image Index is especially helpful.
If you are interested in learning more about research on organic herbicides, here are some Web sites:
Corn Gluten Meal Research Page
Using Acetic Acid (Vinegar) As A Broad-Spectrum Herbicide
Alternatives to Round Up for Management of Poison Ivy
The Gardener's Weed Book: Earth-Safe Controls
The Gardener's Weed Book: Earth-Safe Controls, by Barbara Pleasant (Storey Publishing, 1996; $14.95) describes the role weeds play in the environment and how we, as gardeners, can manage them using ecologically sound practices. The book includes information on dozens of the most common weeds, with illustrations and suggestions on how to manage them.