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.
It's one of the only WWI games you'll ever find, it's not the best game ever, it is based on a war that was markedly unfun, but nevertheless is able to pull it off. Generally balanced maps, nice Classes/weapons, and the community is great. Only downsides are bugs/glitches, and not-too-appealling graphics, these will be fixed though, since it is currently only in early beta.
When 1.2b comes out, this mod will really launch into greatness (and become the new Red Orchestra).