Ryzom

The role-playing game Ryzom takes place in the year 2531 on planet Atys.

Unlike other planets, Atys is one enormous tree, large enough to sustain an atmosphere and multiple ecosystems on its surface and within its immense underground caves created by large networks of branches and roots.

Four distinct humanoid races, or 'homins', came to exist on Atys hundreds of years prior to the game's current timeline (although it is not specified how or why!) These races developed into separate and fairly isolated societies, inside their own favored ecosystem. Interacting benevolently with all four homin cultures, but hostile towards each other, were two far more advanced races: the Kami, the legendary magical spirits of the earth, and the Karavan, the techno warriors from outer space. The Kami are depicted as elusive, chaotic, playful, and highly protective of Atys, while the Karavan are more secretive and have never been shown outside their protective suits or far from their machinery.

In the game year 2481, the homin races were nearly exterminated when the hostile and horrific mutated insect-like Kitins were accidentally released from their home deep within Atys' roots. The surviving homin refugees began working together in 2485 to rebuild a single, mixed society in relatively remote areas. Soon after that came the additional threat of the 'Goo', a spreading infestation that renders areas uninhabitable and drives wildlife mad with rage.

Minimum system requirements: 

CPU: 1 GHz (x86-compatible)
RAM: 512 MB
GPU: OpenGL 1.2 compatible with 64 MB VRAM
HDD: ~7 GB