Recently I have been using "The Better World Handbook" by Ellis Jones. The concept behind the book is that our consumerism makes a difference. Meaning, buying products from certain companies is like voting in favor of that company. This handbook rates the companies we buy from based on the criteria of how well they treat their employees, how safe their products are for the environment and for consumers, etc. Basically, we should buy our gas from BP and avoid Wal-Mart like the plague.
It is really a simple way of making a large impact. I strongly encourage you check out the book, although the website is just as good an contains all of information found in the book. Here is the link.