While I was checking out the recent open sourcing of Delver's graphics engine and level editor (3D dungeon crawler with pixel graphics), I stumbled upon another project created by the same developer, called Shockolate, an SDL2 port (for Windows, macOS and Linux) of the classic gaming title System Shock.
The first version was released very recently, in June 2018. It's Open Source, licensed under the GPL3 and based on the PowerPC source code of System Shock released by Night Dive Studios, Inc. That source code has some missing features that Shockolate aims to add, in addition to an OpenGL renderer and mod support, so that Shockolate will be for System Shock what Chocolate Doom is for Doom.
The game assets cannot be distributed with the project due to copyright, so you will need either an original System Shock CD ROM, or a few files from the System Shock Enhanced Edition release.
Prebuilt binaries are available for download for Windows, macOS and Linux here.
If you've never played Delver, I suggest checking it out as well if you're into dungeon crawlers, its graphics style is really neat and now that the source code is available it's become even more interesting:
Delver is written in Java using libGDX, it's been a few years since I used it but I found it really easy to learn, it's a great alternative to Unity for cross platform game development. Just be prepared for the typical comments mocking you for using Java :)