The trench knife. Beautiful in its simplicity, horrifying in its application. Here's an American version from late 1918.

For all that the Great War is (pretty much rightly) regarded as the first modern, industrialized conflict, it's important to remember that the grim reality of man-to-man combat was still an essential feature. It's true that much of the war was characterized by things like flame-throwers, poison gas, tanks, aerial bombardment and machine guns, but the far more primal efficiency of a blade and a blunt object could not be eliminated entirely.

Close combat during the Great War made use of a variety of things that might prove surprising to those viewing it these many decades later. Apart from the expected bayonets and the trench knives I noted above, there was frequent use of billy clubs, crude maces, and hatchets. Some Irish soldiers carried Shillelaghs; A.O. Pollard records in his memoirs that he once came across a dead German soldier armed with a blood-drenched bearded axe that would have been more at home in the Black Forest of the 10th century than in the French countryside of the 20th. Some experienced trench raiders even chose to train themselves in the use of the sling, as it could be employed silently and its ammunition - in the rubble-strewn hell of No Man's Land, at least - was functionally infinite.

Even these weapons weren't for everyone, though. Many men preferred the always-present simplicity of the entrenching tool (basically a very short spade) that they were required to keep in their packs, and made sure they kept its edges wickedly sharp.

The most terrifying clashes of all took place underground, and not just in the trench-based sense. Most combatant powers frequently attempted to dig tunnels under their opposite number's lines for the purpose of blowing them up with mines, and the delicate, awful nature of this process can scarcely be imagined. To the constant threat of cave-ins caused by accident or the explosion of artillery shells on the surface was added the amazing danger posed by the tunnels being dug by one's enemy counterpart.

These men had to work in as close to silence as could be achieved, to prevent their enemies from hearing where they were and digging over to collapse the tunnel. On those occasions where the tunnels met - whether by accident or design - the fights were just fucking brutal. The British, at least, tended to recruit heavily from Welsh mining villages for these details, so the men involved a) knew their business very well, and b) were huge and angry. Imagine sudden all-out fights to the death in near-total darkness fifty feet below the earth, with shovels, clubs, pick-axes and worse coming into play. It is not a cheering thought.

EDIT: As we're on the subject of strangely old-fashioned-seeming weapons in this period, I should mention that many of the cavalry corps deployed throughout the war (primarily at its outset, when things were more mobile) were literally armed with lances and swords. We may smile to think of it now, but they were still amazingly deadly at the time.

This went on a little longer than I had intended, and I apologize for that. I just find these things interesting :/

EDIT 2: HordeOfDoom, in a comment below, recommends a good Australian film about the war in the tunnels, if you'd like to check it out.