There's practically no way to not miss something here, but I'll do my best.

V-1 buzz bomb: pulsejet cruise missile

A comparatively low cost method for hitting long-range targets without having to mount a bombing campaign with piloted aircraft. The V-1 bombings destroyed nearly as many buildings as the entire Blitz, and caused about 1/4 as many casualties, with less than 1/10th as many sorties.

V-2 ballistic missile

The first exo-atmospheric ballistic missile, it's technology ushered in the space age after WWII. In practice it was a less effective weapon than the V-1 but it was so feared it resulted in massive efforts to both destroy the manufacturing and launching facilities and to use deception to throw off the aiming of the weapons.

V-3 cannon

A multi-chamber extremely long barreled artillery piece designed to have a range of over 100 kilometers, and be capable of hitting targets across the English channel. Similar weapons were used to hit Luxembourg in late 1944 and early 1945.

Sturmgewehr 44

The first modern select-fire assault rifle. Combining some of the characteristics of a machine pistol and a battle rifle, assault rifles fired an intermediate cartridge and had the ability to fire fully automatically or semi-automatically, rendering them useful in close-in fire fights, for use in suppressing fire, and for longer range aimed fire.

Messerschmitt Me 262

Technically not the first jet-powered aircraft to be used in WWII but nevertheless a highly successful interceptor aircraft against Allied fighter and bomber forces.

Messerschmitt Me 163 Komet

A rocket powered interceptor aircraft that was a remarkable design for its time but very ineffective in the field.

Amerika bomber project

A project to build a long-range bomber capable of hitting the US from Germany, several candidate designs were submitted and prototypes built (such as the Me 264) but ultimately the German military decided to focus on fighter production for defense.

Fritz X: radio controlled glide bomb

Along with the Henschel-293 and several other similar devices, these early guided bombs were effective but not heavily used. They presaged the era of guided weaponry that would come after the war and forced the development of electronic countermeasures and jamming techniques.

Bismarck-Class Battleships

Continuing the battleship arms race that had begun decades before, the Bismarck/Iowa/Yamato classes were the culmination of the concept. So feared by the allies that extensive dedicated efforts were put forth to sink both the Bismarck and the Tirpitz.

Panzer VIII Maus

One of several ultra-heavy tank concepts during WWII, weighing nearly 200 tons, with 200mm of frontal armor and a 128mm main gun it would have been a beast on the battlefield. Only 2 prototypes were ever built, though none complete, after the war the Soviets attached the prototype turret to the prototype chassis for display in one of their tank museums.

VT (proximity) fuze

One of the main problems of artillery and bomb effectiveness is fuzing. When a bomb goes off on impact with the ground, much of the explosive force is wasted, ideally you want the impact to occur above ground. The VT fuze essentially used a very special purpose radar instrument to detonate the charge at a pre-set standoff distance from the target (usually the ground). This vastly increased the effectiveness of the explosions, especially against personnel. The technology of the VT fuze was considered incredibly secret and VT fuzed shells were held back from front-line units through much of the war due to the risk of the technology falling into enemy hands.

B-29 superfortress: strategic bomber

Pressurized cabin, nearly intercontinental range, capable of carrying up to 9 tonnes of bombs, with a fully electronic firing control system. These formed the backbone of the Allied strategic bombing capabilities from the end of WWII through the early Cold War, and were the formative equipment for the US Strategic Air Command. Not only the apotheosis of propeller driven strategic bombers but also the first bombers capable of delivering nuclear weaponry.

Nuclear Weapons: (manhattan project)

Primarily a US effort with the assistance of the UK and Canada, it required a considerable fraction of US industrial output for several years. One of the most remarkable industrial and scientific achievements in history, as the first fission weapon was produced only 8 years after the discovery of the phenomenon of nuclear fission. The Manhattan project pursued several different routes to bomb production simultaneously, ultimately succeeding along a few different paths (Uranium enriched from a cascade involving thermal diffusion, electromagnetic separation, and gaseous diffusion processes used in a "gun-type" assembly bomb; Plutonium produced from graphite moderated reactors used in an implosion assembly bomb).

Operation Aphrodite: (use of RC bombers as cruise missiles)

The Allies attempted to use bomber aircraft remotely piloted via radio control from chase planes as extremely overpowered cruise missiles, loaded with 30,000 lbs of high explosive. The intent was to take out the highly reinforced U-boat pens and V-weapon sites. The program met with many technical difficulties and several fatal accidents (including one that took the life of Joseph P. Kennedy, Jr. JFK's elder brother and presumed scion of the Kennedy family at the time).

Mulberry harbour

These enormous floating concrete harbors transformed the sandy beaches of the D-Day landing sites into high throughput ports. Millions of men and supplies were transported using these harbors in Normandy until the port of Antwerp was captured.

Operation Pluto

After D-Day several flexible fuel pipelines were laid right across the English channel into France. It took years to develop the technology to do so, but by the end of the war over 172 million imperial gallons of fuel had been pumped across the channel using the pipelines.

Essex-Class Aircraft Carriers

One of the most important naval vessel designs in history the Essex-class carriers were a significant competitive advantage for the US during the war due to their advanced radar, armament, high speed, fire fighting capabilities, and flight operations advancements. They formed the backbone of the US Navy in the Pacific.

Floating Dry Docks

Originally prompted by the restrictions on fortification building imposed by the Washington Naval treaty the US eventually built enormous steel and then concrete floating dry docks capable of servicing ships closer to battle. The enormous Large Auxiliary Floating Dry Docks such as the USS Artisan were capable of floating any ship in the US Navy, even the massive Iowa class battleships or Essex class carriers.

Norden Bombsight

Merging the use of tachymetry (surveying techniques), analog computers, and autopilot systems the Norden bombsight enabled unprecedented accuracy in daytime bombing. It was developed in great secrecy and was a cornerstone of the Allied strategic bombing campaign. In practice, actual bombing proved to be more than 15x less accurate than what the Norden sight was capable of achieving under ideal conditions, and other operational methods were more effective at improving the accuracy of bombing.

Ultra and the Bombe

Building off of Polish work to break the German Enigma cryptosystem the British undertook the incredible effort to be able to crack all versions of Enigma reliably and quickly, and to do so they brought to bear some of the finest scientific minds of history (such as Turing) and built new devices of unprecedented capability (the early electro-mechanical computer "bombes"). Ultimately the effort became one of the most important aspects of the war, and while probably not changing the outcome of the war overall, significantly advantaged the allies in being able to win the war as early as they did.

Chain Home

The effort to roll out RADAR coverage across Britain and make use of the signals data to facilitate coordinated air defense. Not only one of the earliest and most successful anti-aircraft RADAR deployments but also a pioneering example of unified command and control of air defense forces. It was an important component in winning the Battle of Britain early in the war.

MAGIC

Similar to the British efforts with Ultra against Enigma the US codebreakers faced up against the challenging Japanese cryptosystem "Purple" and ultimately broke it with project MAGIC.

Yamato-Class Battleships

Your standard super-battleship program, ultimately producing only a few ships (Yamato and Musashi), both of which were sunk unceremoniously during the war.

I-400 sentoku class submarine

The largest submarines of WW-II these were also submersible aircraft carriers. They were planned to be used in an attack on the Panama canal but the war escalated too quickly for that to occur.

Fu-Go fire balloons

Carried by the jet stream across the ocean from Japan to the US, the Fu-Go fire balloons were intended to rain devastation on the US Pacific coast, hopefully starting multiple devastating forest fires. In practice only a few of the balloons achieved their missions.

MXY-7 Ohka Kamikaze plane

A piloted bomb-plane, the Ohka was basically a cruise-missile with an onboard human as a guidance system. In practice not many of them actually had the chance to be used operationally, and those that did didn't see much success.

Project Z

An attempt by the Japanese to build a bomber capable of reaching the US mainland. Several designs were made but the project was cancelled along with other heavy bomber projects late in the war due to the lack of any realistic ability to produce and operate them.