Fight in rain soaked trenches or charge over No Man’s Land to battle in the War to End All Wars. Use authentic weaponry in your fight with the enemy, call in artillery strikes to take out foes en masse, and coordinate with your team to gain the final victory on some of the most infamous battlefields of The Great War. Screaming Steel: 1914-1918, previously known as WW1: Source is a total-conversion project being developed by devoted modders for the FPS game Day of Infamy.
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).