HackEd is the main graphical editor application as part of the InkyBlackness project, written in Go. It allows editing of game files of the original System Shock (1994 and compatible) to create mods and fan-missions.
This application is the successor of the deck package of InkyBlackness, which provided the first public editor, even before the source release of the original engine. The rewrite was necessary due to several factors.
The releases page provides the latest binaries for both MS Windows and Linux.
Usage of the editor is documented on its Wiki page.
Answers and further help about modding can be furthermore found on the systemshock.org forums, particularly the Engineering subforum.
Level editing details:
Media editing details:
4K glory showing all the windows:
The project is available under the terms of the New BSD License (see LICENSE file).