But Valve kept some cards close to the chest, declining to reveal digital download numbers. (All of the following statistics were described as Valve's estimates of worldwide lifetime-to-date retail sales.)
Half-Life, the debut effort released by the PC-oriented Seattle-area studio a decade ago last month, put Valve on the map and remains its highest seller to date with 9.3 million units sold at retail, according to statistics released alongside a profile in the November 2008 issue of Game Informer magazine. Gearbox Studios' expansion packs Half-Life: Opposing Force and Half-Life: Blue Shift sold 1.1 million and 800,000 units respectively.
Part of Half-Life's success can be attributed to the phenomenally popular Counter-Strike, which originally appeared as a free Half-Life mod. Sold separately, Counter-Strike sold 4.2 million units at retail in its Half-Life-derived incarnation, while the updated Counter-Strike: Condition Zero and rebuilt Counter-Strike: Source sold 2.9 million and 2.1 million units respectively. The Xbox port of the game sold 1.5 million units.
Half-Life 2, released in 2004, has sold 6.5 million units at retail. The 2006 followup Half-Life 2: Episode One sold 1.4 million units at retail. Half-Life 2 marked the beginning of Valve's practice of releasing its titles simultaneously at retail and via its Steam digital distribution service.
Reports in the year following the game's release pegged Steam sales as roughly equivalent to 25% of retail sales, but that proportion is known to have increased since.
Last year's The Orange Box -- which included new titles Half-Life 2: Episode Two, Portal, and Team Fortress 2, as well as Half-Life 2 and Half-Life 2: Episode One -- has sold 3 million units at retail since release. It was Valve's first product to be simultaneously released for multiple platforms.
The company's most recent game, Left 4 Dead, shipped for PC and Xbox 360 last month; prior to its release, Valve announced the game was seeing preorder numbers far in excess of those for The Orange Box. The company has projected overall sales to be 20 to 30 percent higher than those of last year's release, which would give it retail sales of 3.6 million to 3.9 million units.
Combined, the published retail figures total 32.8 million units sold -- 36.4 million to 36.7 million if Left 4 Dead projections are included.
On their face, they appear to suggest a downward trend in sales from marquee title to marquee title, taking into account Half-Life, Half-Life 2, and The Orange Box (Counter-Strike sales are harder to usefully quantify in this manner due to the game's original mod status and its spread across multiple versions).
However, following the release of Half-Life 2, Valve's pace of releases quickened dramatically. There has been at least one significant new release each year, not including reissues -- with the arguable exception of 2005, which saw the release of Day of Defeat: Source, whose statistics were not released.
Furthermore, and quite notably, the published statistics do not include sales from digital distribution, which is known to be an increasingly weighty subset of Valve's overall revenue -- and, of course, it delivers higher profit margins.
Earlier this year, Valve founder Gabe Newell projected that the company's digital sales of its own games would surpass retail sales of its own games -- an important milestone, and one that is much further away for most triple-A-level developers.
Valve clearly expects that trend to continue, and has gone to great lengths to promote Steam usage: Nearly every Steam-distributed title is discounted during its initial offering, and all Valve-developed games require Steam authentication even when purchased at retail.
Digital distribution penetration likely varies by title. Left 4 Dead, for example, is almost certain to see a higher proportion of sales via Steam than has The Orange Box, whose comprehensive nature attracted many console-only players for whom the full Half-Life 2 series had previously been unavailable.
Because of this, and because of the general lack of more detailed public information, it is unclear how much Valve's unit sales are boosted by digital distribution, but it is undoubtedly by a considerable margin.