Browsing the Good Guide site, I see that their scientists have rated more than 2000 shampoos, 2500 coffee products, 100 jeans brands, 498 cell phone products and 3787 cars -- not bad!
What about for particular products? I checked Diet Coke, which has an overall score of 4.9/10.
It scored 2/10 in Health, because of low nutritional value, 6.7/10 in Environment because "This product has a plastic container that has a relatively low environmental impact," and 6/10 in Society, because the company has social policies that rank it in the top 15% of companies rated by Good Guide. When you click on the various scores, you get more information, but not much. I'm not sure why they say that the plastic container has a relatively low environmental impact -- compared to aluminum cans perhaps? Some editorializing for each product would be good.
Still, Good Guide makes it easy for us to start making greener buying decisions.