Blind Mind Studios, a now inactive game developer, have just released the source code and all assets (minus music) of their commercial game Star Ruler 2. Star Ruler 2 is described as:
Star Ruler 2 is a massive scale 4X/RTS set in space. Explore dozens, hundreds, or even thousands of systems in a galaxy of your choosing, expand across its planets, exploit the resources you find, and ultimately exterminate any who stand in your way. The fate of your empire depends on your ability to master the economy, field a military, influence galactic politics, and learn what you can about the universe.
As the why they released the source code:
This repository contains the full source code needed to build Star Ruler 2, and all secondary scripts, data files and assets required to run it.
Blind Mind Studios has been inactive for a few years now, so we decided to open-source the game as a whole and allow anyone interested in how its engine works to tinker with it.
The source code is released under the very liberal MIT license, while the data and assets are under the CC-BY-NC2 license. You can download the source code as well as get building instructions for Windows and Linux over on Github. If you want to connect with other people interested in Star Ruler 2, check out the discord here.