Gamasutra reports that Valve, creator of the Half-Life series and the hugely popular digital distribution service Steam, has revealed lifetime retail sales figures for the majority of its titles.

The original Half-Life, which debuted over ten years ago, is Valve's biggest seller having shifted 9.3 million units to date. Gamasutra also notes that expansions Opposing Force and Blue Shift have sold 1.1 million and 800,000 units respectively.

Counter-Strike, originally a free Half-Life mod, has sold 4.2 million units at retail, while CS Zero and CS Source have sold 2.9 million and 2.1 million units respectively. The Xbox port sold 1.5 million units.

Half-Life 2 has now sold 6.5 million units, with Half-Life 2: Episode One sales currently standing at 1.4 million units. Orange Box has sold 3 million units at retail and was Valve's first title to be released simultaneously for multiple platforms.

Valve's most recent title, Left 4 Dead, is projected to reach overall sales 20 to 30 per cent higher than Orange Box, which suggests retail sales of 3.6 to 3.9 million units.