For several years, the survival horror classic System Shock 2 was one of the most requested titles on vintage game store GOG. It finally made it onto GOG and Steam in 2013, but one major piece of the puzzle was missing: the original, lesser-known System Shock, released in 1994. GOG has apparently tracked down the rights to the game, however, because the System Shock "Enhanced Edition" is now available for $9.99. It's currently on sale for $7.99, or $5.99 if you already own System Shock 2.
System Shock's gameplay is not, in fact, very much like that of System Shock 2. Beyond the obvious graphical differences, it's more of a straight first-person shooter, with some cybernetic upgrades available along the way. There's less sneaking around and figuring out ways to use enemies' strengths against them, but more wire-frame cyberspace sequences and some delightfully weird features like rocket boots. System Shock also indirectly sets up the story for System Shock 2 — it's the origin story of megalomaniac artificial intelligence SHODAN, whose reappearance in System Shock 2 would be a huge spoiler if it weren't right there in the box art.
The major sticking point for modern System Shock players is the control scheme, which uses keys instead of the mouse to look around. An unofficial patch hacked in mouselook several years ago, but the Enhanced Edition includes it by default, plus more optional control schemes, bug fixes, and support for higher resolutions. (The original game is also included.) If you happen to have a VFX-1 headset lying around, I also recently learned that you can play it in glorious '90s virtual reality. Unfortunately, that sounds actually more painful than anything a rogue AI might do to your pathetic human flesh.