I'm attempting to build a customer support centre for my clients that gives them access to training articles, links, news, and videos that teach them how to maintain their site and gives them updated ideas and strategies in online marketing. I'm wondering if anyone else has built something like this and what open source software you recommend using
Any CMS portal could most likely handle your requirements. It's up to you in terms of what you might be comfortable with on the back end. I always suggest that you try a few of them out first to see if they do what you need. The opensourcseCMS site has many CMS systems installed and ready for you to experiment.