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.
Cooperative Source games can be very irritating to play, due to a lack of levels other than the vanilla Half-Life 2 ones.
Games like Synergy made coop fun this way, but Obsidian Conflict takes it to a whole new level. The amount of community mapping and attachment to the mod itself is striking, and produces a huge amount of maps for all to play.
As well as this, OC handles gameplay very well, and the weapons and changes it adds to the game does not detract from either the balance or atmosphere of the original Half-Life 2 game.
Truly, this is the greatest free cooperative Source game I have played. To not give this a try with friends when you have a Source game to spare would be near blasphemous.
Bravo, I say.