Half-Life: 2

Half-Life 2 is one of the best selling games of all time, and the entire Half Life series is the best selling games to date.

In Half-Life 2, you battle the Combine, Headcrabs, and Zombies to help take back the Earth from the Combine's control.

In this game, as with the last, you play Gordon Freeman, a scientist from Black Mesa who caused the Black Mesa incident, teams up with the Rebels and the Vortiugaunts to fight the Combine and re-take the Earth.

Alyx Vance is Eli Vance's daughter who teams up with you in the entire Half Life 2 series to help stop the Combine and take the Earth back from their brute control.

Half-Life Games as of today:

Half-Life, Half-Life: Source, Half-Life: Blueshift, Half-Life: Opposing Force, Half-Life 2, Half-Life 2: Episode 1, Half-Life 2: Episode 2, Half-Life 2: Lost Coast, Half-Life 2: Deathmatch

Steam Forums (HL2 + Ep 1 and 2)