Obsidian Conflict is a cooperative multiplayer modification set in the Half-Life 2 universe. It features a large amount of new gameplay features on all-new maps specially designed for the mod with focus on cooperative gameplay, as well as over hundred user-made maps following the same formula. In addition to that, You're able to play through vanilla Half-Life 2 all the way up to Episode 2, as well as experimental support for cooperative Half-Life: Source play.
Obsidian conflict in my opinion is a beatiful mod, full of heart-racing action, beautiful cinematics, friendly players, and kick-*** maps. This game deserves an 11/10, for it's massive amount of re-playability, innovative map design, and it's quickly growing content.