System Shock is the prequel to my favourite game of all time, SS2.
Please remember, that this is an old game (it came out at around the same time as Doom2)and runs under DOS.

And yes, as far as graphics are concerned, it's age shows (really badly so).
However the gameplay itself is still right up there with SS2 and Deus Ex. It's kinda sad, that in nearly 10 years no-one else has picked up on this style of play. On the other hand it might be a good thing, not having hundreds of second rate clones scattering the store shelfs, like the pure FPS style games do.

The intro shows, that you are a really good hacker, but get caught anyway. Instead of killing or punishing you, the company goons offer you to work for them.
Your job: Deleting "ethical restraints" from the companies super AI, Shodan.
Your reward: They fit you out with an cybernetic implant to interface with computers using only your brain.
Anyway, you start the game just waking up from a 6 month hypersleep necessary after your implant job.
Now on starting the game, this is where the old skoolers will start sighing and become all dreamy, while the newer guys will wonder what the fuss is all about.

Ok, remember this came out the same time as doom2, but correct me if i'm wrong with a few points.

Now this game had a true 3d environment.
You could jump and duck.
You could climb ladders.
You could lean forwards, backwards and sideways.
You had an inventory.
You could search corpses, cupboards, crates and so on.
You could log into computers, to unlock doors, disable security systems and even pick up little games for your pda.

And lots more that today is nothing special, but then was ground-breaking stuff.

To get on with the story:
You are on the Citadel space station, where Shodan has taken control. Now remember that you were the guy who made this AI into what it is now. And it/she is a badass mother:
Most staff have been killed or turned into mutants.
All service/maintenance/security droids are out to kill you.

Well to cut a long story short: You have to make your way from the medic bay you wake up in to the bridge of the station, to confront and destroy shodan.
And it's a long, tough but fun ride :)

One more thing i should mention is the way the difficulty settings are handled here, as i haven't seen this kind of system since.
If i recall correctly there are 4 difficulty aspects of the game that could be set independantly of each other.
So if you are a doom/quake type of player, you might want to set the combat difficulty really high but set the puzzle setting to 0.
If you're like me and didn't care too much for the matrix sections of the game (this is where you "fly" through a wire-grid environment to pick up software and unlock doors and stuff), just turn that to 0.

To sum this up:
This is the big daddy of System Shock 2 and Deus Ex. Outdated and bettered by those two, but worth a look if you're interested in the history of those games

I think there is a free (and legal) copy of the floppy version floating around on the net somewhere.