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.
This mod is da best coop mod. Also it would be very very very very very (...) good if it had steamcloud,achievements,and etc.
Also i want a map like stranded mod in gmod!!!! or minecraft :P
By the way, there are not so many servers...