Nembo has released 'Unsung', a 10 level set which is World War I based. The levels are very well made. Virtually all the graphics have been changed and along with the new sounds and music give the game a feel of its own. Code changes include gun bobbing, intrart, ingame messages, textured floors/ceilings and a lot more. It all blends together very well and feels more like a new game rather than a mod. There's more info and the download at Mod DB. Also available here. Based off Wolf4SDL code and is Vista/Windows 7-compatible.