WWI Source: Fight in the rain soaked trenches or charge over No Man’s Land to battle in the War to End All Wars. Use authentic weaponry to conquer the enemy, call in artillery strikes to take out enemies en masse, or use chemical warfare to disable your opponents! WWI Source is the first and only World War I game to be developed for Steam. All the current WWI mod teams (‘The Trenches 2’, ‘Fields of Horror’, ‘WWI Source’) have united to develop the best World War I game to the Steam community, using the latest Source engine. Please visit the official website for more information on WWI Source and the team.
WWI Source Alpha gameplay Video
As it says this is a alpha build of the mod.