Half-Life 2

"Half-Life 2 is a science fiction first-person shooter computer game and the sequel to the highly acclaimed Half-Life. The game uses the Source game engine, which includes a heavily modified version of the Havok physics engine and was also the first video game to require online product activation.

Half-Life 2 takes place in a dystopian world in which the events of Half-Life have fully come to bear on human society, which has been enslaved by a multidimensional empire known as the Combine. The game takes place in and around the fictional City 17 and follows the adventures of scientist Gordon Freeman who must fight against increasingly unfavorable odds in order to survive."

Source: Wikipedia, "Half-Life_2," available under the CC-BY-SA License.