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.
If you own the Half-Life anthology (or just Half-Life 2) and havn't played this mod...WHY NOT?! DOWNLOAD IT NOW, SERIOUSLY.
New weapons, extensive content mounting and supported campaigns from the entire Half-Life 2 series (Including HL:Source!). Custom scripts support for creating your own weapons, loads of great custom maps and features.
You won't be dissapointed!