Annenberg Learner is usually my second stop (after PBS Learning Media) when I’m looking for content for a new unit. It has fewer resources, but they are just as good. Learner.org focuses more on professional development, so if you’re looking for some advice on how to teach literacy to middle schoolers, this is a good resource. But their actual classroom content is excellent. They have many videos and some really great simulations – just try their amusement park physics. Content is available for K-12 and it’s free.