Windows/Mac/Linux: Khan Academy is very useful for learning about computer science, maths, and many other subjects. If you'd rather learn without being tied to an internet connection, KA Lite allows you to manage and view downloaded Khan Academy videos.
The app works across platforms and you can sync your devices with KA Lite's servers so you can set up one machine and have the changes affect several machines. This is particularly handy if you're preparing a classroom environment. You can download videos for offline playback and access them locally via the browser. KA Lite requires Python, and will prompt you to install it if isn't available.