Through the Mud and the Blood by TooFatLardies. I've really enjoyed Sharpe Practice, and M&B uses basically the same mechanisms. The rules look like great fun, and also contain a great intro to Great War infantry tactics as they evolved from 1914 thru 1919.
Planning on using M&B for some RCW/Interventionist/Dunsterforce entertainment, at least for now, although visions of trench tiles dance in my head!