Every war has its secret weapons, and probably none had more than the Second World War. Even at the time, the German love of "wonder weapons" was well-known, and made a fine psychological weapon in itself. So did the Reich's love of science and engineering. Allied planners and soldiers never knew what kind of monstrosity they'd encounter from battle to battle, or what new horrible fruit would drop from the twisted tree of German science. In short, we should be glad some of these World War II weapons never saw the light of day.
Maintaining that fear, and thereby keeping the Allied forces on their toes, was a prime concern for Germany during WWII. So it's no surprise that the Nazis far and away dominate any list of the secret weapons of World War Two. Still, we had a few of our own, not least of which was the atomic bomb, which effectively ended the war.
Good thing it did, too. Because as you'll see, more than a few of these devastating secret weapons of WW2 were just inches from operational when the war ended.
Landkreuzer P1000 Ratte
If the Germans had finished building it, the 1,000-ton Ratte would have been the largest conventional tank ever built. The Nazis envisioned the Ratte as a kind of battleship for land, literally. Its massive 283 mm cannon was to be taken directly off the deck of a Scharnhorst-class battleship, and would have been capable of hammering hard targets up to 17 miles away. With armor 14.2 inches thick in some places, the Ratte would have been almost invulnerable to all but the biggest bombs in the Allied arsenal.
The A9 was a variation in the V2 rocket that never quite got off the ground. And good for us, too. As you might have guessed from the name, this two-stage rocket was intended to cover the distance between America and Europe, and deliver 1,000-pounds worth of payload as far west as central Pennsylvania. Germany never could get the guidance systems to work quite right, so they opted to use U-Boats parked offshore to guide the missiles in on final approach. The missiles would likely have been in operation by 1946 had the war continued.
The Me 264 (Messerschmidt's follow-up to the jet-powered Me 263) was the most favored of the designs submitted for Hitler's Amerikabomber program. A fairly conventional design not unlike our own B-29, this bomber could have carried a respectable 6,600 pounds on the 9,000 mile round-trip journey to America and back, or a massive 30,000 pounds 5,300 miles. The latter might well have been disastrous for us had Germany captured landing and refueling fields in Greenland or Iceland. But by the time Germany worked the bugs out, they didn't have the time or materials to build it.
You can think of the Sturer Emil as a kind of sniper rifle for tanks, and the one that did see action in Stalingrad had racked up at least 22 kills before it was captured. Essentially a 5-inch anti-aircraft gun mounted on a Henschel VK30.01 tank chassis, the Emil was a massively effective and fairly inexpensive weapon. Only material shortages and the end of the war kept Germany from putting it into mass production.