I thought that article from GoldStandard.org was good, but we just published an article that goes much deeper and talks about everything you need to know about offsetting your carbon footprint.
We also answer the following:
How can I measure and reduce my carbon footprint?
What role do trees play?
Can I plant enough trees to offset my carbon footprint?
What makes it hard to completely offset my impact?
We quote 28 different sources in the article -- it's quite authoritative.