A quarter-century is a long time to wait for the release of a mod, but that's how long it took for System Shock: ReWired to appear on the scene. It's "the very first full fan made singleplayer campaign for the original System Shock," according to its ModDb page, featuring three all-new levels, four new cyberspace areas, new puzzles and audio logs (no voice acting, though), and a brand-new ship to explore: The hospital ship UNN Bismarck.
ReWired hasn't been in development for 24 years, of course—it actually took about four months to take. But it claims the distinction of being first after all this time because it's only recently that modders have been able to do this sort of thing. Nightdive Studios, which is currently working on both a remake of the original System Shock and a brand-new game in the series, released the System Shock source code in April. System Shock itself came out in 1994—September 23, to be precise.
Installing the mod is a matter of "six easy steps," as creator the-3rdplayer explained, apparently without irony. It involves replacing some files, a process that isn't exactly brain surgery but that can be a little intimidating if you haven't done such things before. Take my advice: Do not overlook the importance of step 1. Obviously you will also need System Shock if you want to give the mod a try, and this would be a fine time to grab it—the Enhanced Edition is currently on sale for $1.50 at GOG.