This is good for those interested in what the different vitamins are and how much of them different brands contain. However, if you're wondering what they do, then this won't be too helpful. You can link to the scientific papers that talk about efficacy, but these papers aren't a very easy read, and usually it's just an abstract, not the full paper.

This resource is good as added brand information to the other top two links here, but not as good for most people on effectiveness because it cites often difficult-to-read scientific studies, rather than translated summaries of results. Worth a look if you want to know for sure whether your supplements are balanced, but it'll take some effort to figure it all out.