Team Fortress Classic
Valve enlisted Gearbox Software to develop three expansions for "Half-Life": ' (1999), ' (2001) and "" (2001). Valve acquired TF Software in 1998, a group that had made the popular "Team Fortress" mod for "Quake", and remade the mod for GoldSrc as "Team Fortress Classic", released in 1999. Valve released the software development kit (SDK) for the GoldSrc engine, facilitating numerous user-created mods. Valve acquired the developers of one popular mod, "Counter-Strike", to create a standalone "Counter-Strike" game. Happy with Valve's success, Harrington left in 2000.