How do you play multiplayer in Half life

There is something called Decay on the first half-life where you play as two women during the incident. There is also a half-life 2: deathmatch, and that's more or less it.

There is Half-Life Deathmatch and Half-Life Deathmatch: Source. You can also download a mod, "Sven Co-op", which allows you to play the Half-Life story with other players.