HalfLife2.net have kindly collated some lifetime retail sales figures of Valve's Half Life universe.  Still no sign of Steam digital content sales figures though.  Gamasutra has more info.

Half-Life: 9.3 Million

Half-Life Opposing Force: 1.1 Million

Half-Life Blue Shift: 800,000

Half-Life 2: 6.5 Million

Half-Life 2 Episode 1: 1.4 Million

Counter-Strike: 4.2 Million

Counter-Strike Condition Zero: 2.9 Million

Counter-Strike Source: 2.1 Million

Counter-Strike Xbox Port: 1.5 Million

The Orange Box: 3.0 Million