Baldur's Gate II: Shadows of Amn
Created by Black Isle Studios and released in 2000, Baldur's Gate 2 remains one of the most popular classic RPGs (http://www.gog.com/en/catalogue#all_genres/price/999/order/bestselling/). Set in the Forgotten Realms and using the Advanced Dungeons and Dragons ruleset, the player takes on the role of a Bhaalspawn, a child of the god of murder, and must unravel the plans of the antagonist Jon Irenicus.
Baldur's Gate 2 supports multiplayer campaigns over LAN or internet, though the easiest way to play online maybe using the Game Ranger free program.
It is easiest to install and run Baldur's Gate 2 on a Windows operating system, but it is possible to play Baldur's Gate 2 with a Linux OS using Wine.