Half-Life Coop is a mod for Half-Life 1 that adds a cooperative mode to the game's campaign. Unlike other coop mods like Svencoop or DC coop, it does not change important aspects about the game, because the objective of this mod is to be compatible with the servers of Half-Life 1 and keep it as vanilla as possible.
This mod is still in process to add more improvements for cooperative mode. This one does not contain the final boss (Nihilanth)
Download the file, extract it, enter the extracted file and read the "Installation" file for more information.