Pale Moon extensions > Greasemonkey >
A cross-browser script that adds a button to download YouTube videos as MP4 and FLV files. This script downloads the videos directly from YouTube and it integrates with YouTube's interface.
It creates a button under the video, to the left of "Add to"
Online learning >
An offline version of Khan Academy.
A free lightweight solution providing high-quality education where you need it most.
Though this software is unofficial, it is known of and approved by the official website.
disliked - because it has no concept of building a larger topic of things to work on, and showing an easy path to progress through them. Needing to pick through a large number of topics, sub-topics and quizzes and then manually and repeatedly going back into that list is inexcusably frustrating.