How about the allied landing craft as underrated weapon of war? If I have to get specific, I’d have to say the LCVP or Higgins boat; but LCAs, LCIs, LCM, LSTs, etc also played integral parts in winning the war. Over 20,000 Higgins boats were built during the war and they were the work horses of the allied landing operations over the course of the war.
No less an authority than the Supreme Allied Commander, Dwight Eisenhower, declared the Higgins boat to have been crucial to the Allied victory on the European Western Front and the previous fighting in North Africa and Italy:
Andrew Higgins … is the man who won the war for us. … If Higgins had not designed and built those LCVPs, we never could have landed over an open beach. The whole strategy of the war would have been different.[2][3][4]
The Higgins Boat was used for many amphibious landings, including Operation Overlord on D-Day in Nazi German occupied Normandy, and previously Operation Torch in North Africa, the Allied invasion of Sicily, Operation Shingle and Operation Avalanche in Italy, Operation Dragoon and of course the Pacific Theatre at the Battle of Guadalcanal, the Battle of Tarawa, the Battle of the Philippines, the Battle of Iwo Jima and the Battle of Okinawa.
http://en.wikipedia.org/wiki/Higgins_Boat
The invasions of North Africa, Sicily, Italy, Normandy, and numerous Pacific campaign landings would not have been possible without the plethora of landing craft designed for the purpose. Not only were they crucial in landing the assault forces right on the beaches, they were capable of bringing in much needed supplies to sustain the invasion and evacuated countless casualties to hospital ships. Hitler and German high command did not believe that the allies could supply a large invasion force over an open beach, they were wrong. Despite not possessing a sizable port in Normandy, the allies were able to not only sustain their forces ashore but also buildup men, equipment, and supplies for the breakout as well. Heck, you know it was crucial to the war effort when no less than Eisenhower sings it’s praises.