Half-Life 2: Episode One launches
Valve released Half-Life 2: Episode One for Windows on June 1, 2006, pioneering short-form episodic sequels that continued Gordon Freeman's story straight out of City 17.
Source: Valve / Steam news via store.steampowered.com — 'Episode One, released at retail outlets and via Steam on June 1st'; also Altered Gamer gaming-history listing for June 1, 2006