System Shock 2 is, in my humble opinion, one of the most enjoyable games ever created. There really hasn't been a game that matches it yet, though the original Deus Ex and Bioshock both have similarities.
It's a blend of first-person shooter and role playing, with lots of creepy-scary thrown in. The player must specialise and work out their own solutions, which makes for high replayability.
Unfortunately, System Shock 2 is so old people can have horrible trouble just getting it to run properly on a modern computer. This page is here to help!
System Shock 2 no longer appears to be available via retail. Your only options are therefore either buying a second hand CD via eBay or the like, or simply procuring the files via whatever means you have access to.
System Shock 2 is old, and the visuals suffer for it. Fortunately, there are lots of fan-made improvements! The install method outlined above prepares the game to use these mods.
Specifically, note that the game has a new folder called DMM and a new exe called ss2bmm.exe. To use a mod, extract the contents of the zip file into this folder, then run ss2bmm.exe and select it to be active. You can choose the order mods are applied in, so if two mods change the same item you chan change which one has priority.
There are LOTS of mods available! A handy list of the best ones to use is on this thread at www.systemshock.org. It has everything else you need to know, and I suggest you try all nine that it recommends. The bloom settings are nice too.