Source code for Half-Life mod Natural Selection released for free
Unknown Worlds has released the source code for Natural Selection but it's not entirely 'open' source.
Developer Unknown Worlds has released the source code for Natural Selection, the Half-Life mod released way back in 2002.
The team noted that it's not entirely open source, but you can grab it from GitHub if you want to tinker, improve or generally mess around with its internal organs. It'll give you something to do until Half-Life 3 is released...
Unknown Worlds is currently working on RPG/sandbox hybrid Subnautica.