Originally released in 1999, System Shock 2 was mind blowing at the time, and far beyond anything else on the market at the time. While today the graphics are not much to look at, the gameplay and story are still far beyond most of the other games out there.
If you are wondering what this game is like, it is a lot like the first Bioshock. However, it is much harder, and has actual RPG elements, not just the window dressing of them. "Deus Ex" might be a better comparison now that I think of it. The point being that it is a first person game, with guns, and with lots of stats and skills to keep track of, that allow you to approach the same situations in many different ways. This leads to a very high replay value.
So pick it up on GOG http://www.gog.com/gamecard/system_shock_2. Or wait for the Steam release. Either way, you owe it to yourself to play this masterwork.