synergymod.net/

Synergy is, as the name suggests, a cooperative Half-Life 2 mod. Synergy was previously known as two separate mods, "DC Coop 2" and "DevCoop". We merged and are now bringing you the best cooperative experience possible.

The main objective in Synergy, is for all players to complete the given objectives for each level; Escape, Assault, Defend, etc. You work together as a team to finish the level.

Synergy also supports Deathmatch and Teamplay (Combine vs. Rebels). We encourage the use of NPCs in all levels, even if it's Deathmatch. We try to give the level artists much more customizability than Half-Life 2 already provides.