A lot has been said about the value of data viz, but the folks at R2D3 have truly taken this to a whole new level by using very sophisticated but also very intuitive data viz techniques to teach the basics of machine learning. I was really blown away by the way the step-by-step visualizations on this page lead the reader through all the intuitive steps to arrive at a pretty clear understanding of machine learning, in this case focusing on decision trees.
If you are an experienced practitioner you will enjoy the sophistication of this presentation and if you know someone just starting out and interested in a clear and intuitive introduction to the topic you would be hard pressed to beat this.
I know you will enjoy this. See it here. A Visual Introduction to Machine Learning