Desiring God (desiringgod.org) is the ministry of John Piper, founded on the conviction that 'God is most glorified in us when we are most satisfied in him.' The website makes everything free — thousands of sermons, articles, devotionals, Q&A responses, and book chapters — on the explicit theological principle that the gospel should be given away freely.
For SRE teachers, the practical value is this: whatever passage or topic you're preparing to teach, there is almost certainly a Desiring God article or sermon on it that will deepen your own understanding and give you language and illustration you can adapt for your lesson.