Almost every know about Khan Academy. You can sign-up and learn a variety of subjects in a safe environment on your own time. But they also have a feature that allows teachers to set up classes so that you are then able to track what your students worked on so you know what they are learning. So you can integrate their already made videos (and short snippet of info), into your curriculum, or you can have them use it as an alternative learning source. Students can earn badges for completing tasks, so you can have them compete in a fun way to learn topics you want to cover.