Khan academy Helps understand and practice challenging topics in ways you see fit for yourself. It also gives you flexibility on the content that you learn. Making it useful for education.

The ability to assign lessons/ see progress

Everything! Khan Academy is a plethora of resources and courses for everyone from elementary to college prep. It gives a thorough explanation in short mini-videos and then short practice assignments to determine mastery. My students love it because there are only 4 questions in each skill activity and they can go back and watch the videos as many times as they need to.