The Elder Scrolls III: Morrowind

Morrowind allows you to create and play any kind of character imaginable. You can choose to follow the main storyline and find the source of the evil blight that plagues the land, or set off on your own to explore strange locations and develop your character based on their actions throughout the game. Featuring stunning 3D graphics, open-ended gameplay, and an incredible level of detail and interactivity, Morrowind offers a gameplay experience like no other.

OpenMW is an attempt to reimplement the popular role playing game Morrowind. It aims to be a fully playable, open source implementation of the game. You must own Morrowind to use OpenMW.

Minimum system requirements: 

The Elder Scrolls III: Morrowind data files and the OpenMW engine