Obsidian Conflict mod for Half-Life 2

Obsidian Conflict is a cooperative mod for Half-Life 2. It not only allows you to play all the way from Half-Life: Source to Half-Life 2: Episode 2 cooperatively, but features a large amount of new gameplay features and user-made cooperative maps.

-Offers all Source engine Half-Life series games playable in co-op online (Half-Life: Source, Half-Life 2, Lost Coast, Episodes 1 & 2)
-A large set of included custom maps, some in the form of classic combat & teamwork puzzles and others offering extraordinary new gameplay experiences such as cooperative farming and fishing
-New weapons, including dual Uzis, Medkit, Combine Sniper Rifle, the Alyxgun, the Tau Cannon, deployable Manhacks, as well as support for scripted weapons allowing use of weapons from Counterstrike and Day of Defeat Source
-New NPCs, including many from the orginial half-life such as a fully-functional ichthyosaur, as well as a working mortarsynth from HL2
-Special merchant NPC and related entities, offering gameplay where players can spend points from their score to purchase weapons and map-specific objects
-Many co-op related features, such as a server-side lives system, hud waypoints in the form of a player locator, attention & medic calls, and map objectives, as well as players' inventory carrying over between level transitions
-Great support for community mappers, including features like the mapadd system to easily edit existing maps without having to recompile them, as well as tutorials and prefab examples for all of OC's new entities and features