Let's see...I've married weapons to tactics, as new weapons and new techniques are mostly inseperable.
World War I
1.
Machine gun - never-before-seen firepower decentralized to battalion and company level
2.
Infantry mortar - while mortars had been around before, the light mortar was an incredible innovation: now the infantry can bring their own artillery with them!
3.
hydraulic-recoil artillery - These guns did not have to be repositioned after firing, so their effective rate of fire and accuracy were phenomenal. Also, high-angled artillery could fire from impunity behind the "dead ground" on the reverse-slope of hills and such. This wasn't started in WWI, but it became mainstream practice there.
WWII
1.
Atomic bombs. I think they're overhyped myself, but they do present destruction on a mass scale. Caused a revolution in geopolitics
Postwar
1.
Tough, reliable infantry assault rifles - the AK series comes to mind.
2.
Airpower as a major factor in war - America in particular has a tendancy to only go to the ground after they've secured air superiority.
That's six. If I forgot anything major, I'll add it in.
