Released on November 19, 1998.

Valve invites you to experience the Half-Life 25th Anniversary Update
YouTube - Half-Life: 25th Anniversary Documentary on 2023-11-17 by Valve.

HLMaster A Half-Life master server…
Steamless Project

WON2 status
Half-Life 1 Deathmatch - WON2 Gaming

Bots United - Filebase for download of several different bots for Half-Life.

Mod DB - Xash3D Engine has been tested for compatibility with many of Gold Source games and MODs. At this moment it should provide you an ability to successfully complete almost any existing singleplayer Half-Life mod without compatibility problems. Some mods can be not totally compatible, if they are too hardly modified or using some very specific or unstable features of Gold Source engine.
Mod DB - List of singleplayer Half-Life mods tested under Xash3D

SvenCoop
Natural Selection, released on GitHub:

• unknownworlds - Original
• fmoraw - Linux Build
• fmoraw - Visual Studio 2010 Rebuild

Half-Life Creations - Hosted Sites

Metamod
Metamod-P
AMX Mod X is a versatile Half-Life metamod plugin which is targetted toward server administration. It has a wide array of scripting capabilities so people can write “plugins”.