A handy, all-in-one cheatsheet for Linux and UNIX environments. If you need to look up how to do something fiddly, this is a good place to look first.
The Philharmonia Orchestra has released samples from several dozen instuments under a Creative Commons license (CC-BY-SA). Commercial works which use these samples are explicitly approved.
Google has open sourced their Material Design Icons Library. It includes icons in PNG, SVG, and CSS glyph formats for many devices, functions, and features. Spritesheets are included. The icons have a Creative Commons By Attribution-Share Alike v4.0 license, so feel free to download and use them.
A free and libre textbook of OS design and implementation on the x86 architecture. The full book can be read here or downloaded as a PDF from this site, or you can check out the book from its Github repository (https://github.com/littleosbook/littleosbook).
A massive corpus of annotated and tagged ngrams for use in machine learning and NLP. Free to download. It'll take time to grab it all because it's so finely split up. Creative Commons licensed (BY-NC-SA v3). Relationships between parts of speech and words are broken out, also.
A security training manual for activism and human rights defenders.
Archive of shortwave radio recordings to download.