Valve says Source 2 will be "available for free to content developers," pointing to similar recent announcements about Epic's Unreal engine and the Unity engine.

That might change, though, given the hard work of [PortalRunner]. Quite literally, [PortalRunner] hacked the Source engine and Portal 2 to actually run a working HTTP web server.

Files found within Valve's Source Filmmaker program reference a next-gen engine from the company, referred to as Source 2. Numerous lines of code reference Source 2, as uncovered by Valvetime.net ...

The next-gen Source engine is here, 11 years after the first version shipped with Counter-Strike: Source and Half-Life 2 ...

Valve's game engine perfectly highlighted the transition between old and new design sensibilities.