A free program for Windows, by yamashi.
Cyber Engine Tweaks is an open-source framework for creating custom modifications to the in-game assets. Its goal is to make it easier for developers to create new mods and mods for existing games. It provides an API to access the in-game data and it also includes a UI to configure the different mods you are using or using the console directly.
It's a very simple tool that's meant to make the life of a modders easier, it can be used on any platform that supports the API.
It can be used to create custom modifications to the in-game assets, however, the framework's focus is on the game's assets, not the game's code.