As Gordon Freeman, a young research associate in the Anomalous Materials Laboratory of the Black Mesa Federal Research Facility, your mission is to investigate a strange crystalline being. You find yourself battling not only the alien monsters but also the government troops sent in to keep the crisis under wraps. Sophisticated monsters and creative technologies make this game a winner.

Minimum system requirements: 

CPU: 2.8 GHz dual core
RAM: 1024 MB
GPU: nVidia GeForce 8600/9600GT, ATI/AMD Radeaon HD2600/3600
AND: Steam

External links:
Developer: Valve