I'm not a doctor, but I am a medical student, and one of the physicians I work with just recommended consumerlab.com. This is an independent lab that tests various vitamins and supplements to see what's in them.

Another resource I use is the Complementary and Alternative Medicine Guide at the University of Maryland. The site isn't the easiest to use, but it's good for finding out, say, what are the standard medical and alternative methods of treating arthritis.

There's also NutritionMD.org, website of the Physicians' Committee for Responsible Medicine. This site has dietary recommendations for various common conditions. If you'd prefer a hard copy, they put out the Nutrition Guide for Clinicians, something I use weekly.