one of the first fully 3d shooter / RPGs, with skills and inventory and plot and such. really groundbreaking, and a great story and immersive environment to boot. Ultima Underworld may have been earlier, but most of the other games were either pure shooters like doom, or pseudo-3d RPGs like Dungeon Master and the Wizardry series.
Same guys behind it were the ones who made Thief, so that should tell you something about the creativity and quality of story telling. I heard a rumor that the founder was up to something new, but that must be bullshit since his last company was bought by zynga back in 2011 ):
Fans keep updating this game to be compatible with modern computers, I try to play it every couple of years but it always crashes sometime in the first or second level, and I'm not that patient. I hear some people have it working smoothly, and the graphics have been updated for higher resolution screens (I dont think the textures are improved, just higher display modes are supported now). I couldn't get it going last year, but you might have better luck than me.
This post was updated like five days ago, so someone is still maintaining it:
http://www.systemshock.org/index.php?topic=211.0