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