Obsidian Conflict (AppID: 17750)

Key features

• 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 with a great variety of gameplay styles, from classic walkthrough combat and deathmatch, to teamwork puzzles and lives mode, or even abstract game modes like fishing or farming.
  
• New weapons, including dual Uzis, Medkit, Sniper Rifle, the Alyxgun, the Tau Cannon, deployable Manhacks, as well as support for scripted weapons allowing use of weapons from Counter-Strike and Day of Defeat Source.
  
• New NPCs, like the Mortar Synth and support for NPCs from all the Half-Life games like the Gonarch or the Hunter.
  
• Many co-op related features, such as lives system, HUD waypoints, player locator, attention & medic calls, merchants, score 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.