Garry's Mod, the fan-created sandbox editor for Valve's Source engine, has sold 312,541 units since it went on sale two years ago through Steam.
UK-based lead developer Garry Newman divulged the figure by way of his blog.
First released in 2005 as a free mod, Newman updated and re-released Garry's Mod through Steam in 2006 for $9.99. At the time, he publicly revealed certain terms of the publishing agreement, which saw an even revenue split between Valve and Team Garry. The game's site lists eight developers, including Newman himself.
Assuming constant pricing for two years, the game would have generated roughly $3.12 million in sales.
Such data is exceedingly rare for Steam-distributed games. Though Valve recently released retail sales figures, it has never publicly given digital distribution numbers.
Garry's Mod allows a wide variety of user freedom with the assets and systems that underpin Valve games such as Half-Life 2, Counter-Strike, and Team Fortress 2.
Users equipped with the software have spawned machinima, amusing facial distortion of Half-Life series characters, virtual Rube Goldberg-like devices, and even a long-running online comic series.