Here is the OFFICIAL patch for System Shock 2. This will upgrade it from the original install of 1.15 to 2.3.
shkpatch.exe-5.23MB
According to Irrational Games website
"This patch implements cooperative multiplayer for System Shock 2. Up to four players are supported over internet or LAN. In addition, several bug-fixes and additional features are implemented including the ability for the user to modify weapon degradation and monster re-spawning rates. This patch is only intended for the original release of System Shock 2. If you have purchased a re- released "budget" edition, you should not need this patch."