If you already know this game, you should require no introduction. If, for some reason, you haven't played it yet, you've somehow sidestepped a legend and should atone by experiencing it at your earliest convenience.
Arcanum is a computer role-playing game and at the same time the name of a unique world - a classic fantasy setting at its core, but intertwined with steampunk, with the world currently in the middle of an industrial revolution. Its creators took advantage of this by implementing a beautifully complex character creation system and having the world and storyline adapt to whatever character you have created. To top it off, you can travel the open-ended world as you see fit and complete quests in a variety of ways - from combat, through diplomacy, to plain thievery. It's simply one of the greatest RPG titles of all time. What more recommendation could you possibly need?