He is one of the few people I know who has an insanely prolific online presence. His blog, What's New (https://terrytao.wordpress.com/) has hundreds of detailed articles on various math topics and occasionally I find myself using it as a beginner's reference to detailed topics. He's also published his blog posts as books. It is astounding how one person can produce so much intellectual output! This is in addition to his mathematical papers and books (https://terrytao.wordpress.com/books/)