The collaborative, web-based, massively multiplayer game Glitch began its initial private testing in 2009, opened to the public in 2010, and was shut down in 2012. It was played by more than 150,000 people and was widely hailed for its original and highly creative visual style.
The entire library of art assets from the game, has been made freely available, dedicated to the public domain. (CC0)
Github repo for a book that teaches you how to write web apps using Go.
The Internet free music archive. Just what it says on the tin.
The official website for a transhumanist-themed RPG which is light on the rules but heavy on the "Wow, my character can do that?!" factor.
The book that started the Creative Commons movement.
A far future RPG in which the players play field agents keeping the peace between a dozen civilizations by any means necessary throughout the universe. Also has strong themes of political intrigue, wonder, and responsibility. Published under the Creative Commons v3 license, though you can purchase PDFs of it from DriveThruRpg.com or buy dead trees from lulu.com if you want to support the project.
A Creative Commons-licensed book by Al Sweigart that teaches programming in Python through writing your own video games. Full source code is included.
A CC-licensed cheatsheet for building with and programming the Arduino.
ccMixter is one part archive, one part social network of musicians who make their work freely available under a Creative Commons license.
The DIYLILCNC is a fully functional, open source 3-axis CNC that you can build with basic tools and parts that can be locally sourced. The idea is that you develop a 3D design in a CAD application, put feedstock into the CNC, and print your design to it, and it cuts and grinds away everything but what your design is supposed to be. Total cost of construction is about $700us. You can download the plans and DXF template files from the website for free (they have a CC-BY-SA license).
Wikihouse is an archive of open source building designs contributed by engineers and architects around the world. Each design is peer-reviewed and analyzed by others to ensure that the designs are sound. Some of them have already been built and the results posted for further review. The design principles include being as easy to build by people with minimal formal training, as energy efficient as possible, and easy to modify to fit a particular purpose. All designs are made in Google Sketchup and can be cut with an automill if available.
A pair of books licensed in the Creative Commons (BY) which talk about the architecture of open source software. The core developers of four dozen projects talk about how their software is structured, and most importantly why. They were written so that F/OSS developers wouldn't have to learn by reinventing the wheel, instead there would be a reference.
There are two books, which you can read online and download for free. Or you can buy them as Kindle editions (the proceeds go to Amnesty International).
The Universal Construction Kit is a set of open source components that you can fab on a 3D printer, which allows the user to hook together different construction kits without duct tape or twist-ties. The components make it possible to use Lego, Duplo, K'Nex, Lincoln Logs, and other construction kits in the same project simultaneously.
The whole set can be downloaded as a .zip file from the website.
The .stl files carry a Creative Commons By Attribution/Noncommercial/Share Alike v3.0 license.
Note that the .stl files are in inches, not mm. Double-check the settings of your 3D printer before fabbing!
A handy, all-in-one cheatsheet for Linux and UNIX environmetns. If you need to look up in a hurry how to do something fiddly, this is a good place to check first.
Rice University is releasing under a Creative Commons license college level textbooks written by professional textbook writers. The texts have passed through a peer review process as well for vetting and fact checking.
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.