Shadowrun Toolbox is an open-source application mainly to manage your Shadowrun character sheets. It doesn't replace the rulebooks, you still need them to play the game. The tools just make the not-so-interesting things such as filling in the character sheets, easy and fast so there is more time to play the game.
Currently the tools are written for Shadowrun 5th Edition, but tools for other editions, older and newer, can (and probably will) be added later.
The application runs on a variety of operating systems including Windows, Linux and MacOS X because it's written in Java which makes the application operating system independent.
An older version of Shadowrun Toolbox targetting SR4 exists: SR4Toolbox which can be found on its own website.