I was quite surprised by the existence of the war scythe, which is not actually as scythe-like as one would think, but it actually works in combat this time round. The blade's upturned so it looks much like a glaive, save that the blade is on the inside edge of the curve, rather than outside. It seems to have been popular in Eastern Europe, where of course agricultural scythes were more commonly used. It doesn't look terribly effective to me, but it's better to have than a simple spear in most cases.
Apparently there's at least once instance of German soldiers in WWII stringing piano wire across roads at neck height to injure or kill Allied soldiers riding in the oft open-topped Willys MP; I'm not sure how truthful that can be taken to be, but war is a time for such unlikely happenings. Many British devices of the same war were unusual, but I can't think of any off the top of my head aside from an inflatable tank, but that's for deceit and distraction, not a weapon.