Despite the presence of oil, the First World War was pretty steampunk. You've got mid-late 1800s strategy and tactics (and even military fashion) meeting (then) high technology featuring fleets of airships in the sky, cantakerous early warplanes, riveted-steel early tanks (which were even called Landships back then), heavily armed & armoured steamships, monstrous pieces of artillery, armored trains and many other bizarre equipment that would not have been out of place in a steampunk piece of media. The war was also extremely Diesel Punk because it saw the advent of industrial warfare, modern military tactics, a new type of world spanning the globe, and new types of society. As such, in many ways it is the border between the two punks, and a useful case study in how the two can be blended.
Airships and airplanes have roots in Victorian-era steampunk. Henri Giffard's airship, the first powered aircraft in the world, flew in 1852 using a steam engine. The Eole bat-steam-plane flew in 1893. Barely.
There was a little bit of Steam Punk in Ancient Greece. One of the most famous examples is Hero of Alexandria who built, among other things, a primitive steam engine, a wind-powered organ, a vending machine that dispensed holy water, force pumps for fire engines, and a hydrostatically powered fountain. Some historians today actually debate why the Ancient Greeks did not have an Industrial Revolution; a leading theory is that the abundance of cheap slave labor and a corresponding shortage of easily obtainable fuel served as a disincentive.
Steampunk Prostethic arm works using rocket-style motors that run on steam. One of the stranger side effects this will have when in production is that the excess steam will be vented off as sweat.
Even today steam enginesnote They're steam turbines instead of older, less efficient reciprocating piston engines, but they're still engines driven by steam. drive submersibles, tremendous ships of war, and power cities. But since the water is boiled by radioactive isotopes and not by coal or wood, we tend to call them "nuclear reactors" nowadays. There's a reason they called the first Atomic Sub the Nautilus!
What people hardly realize today is that fact that many weird inventions in Jules Verne's novels were based on actual technologies of his time, blown Up to Eleven and therefore pretty hard to be put in practice. As Nautilus had been based on the Real LifePlongeur (1863-1872), which it resembled in description, but which also performed poorly (a top speed of barely 4 knots, while the battery-powered Nautilus was said to make 50 knots(!), Robur the Conqueror's 150mph automobile from 1904 Master Of The World had been based on Real LifeJamais Contente racecar from 1899 (again electrically-powered) and so on. And, strangely connecting Steam Punk and Diesel Punk ages together in Real Life, the British Great Western Railway ran from 1838 to 1892 high-speed trains on 2,140mm gauge tracks, prefigurating Adolf Hitler's plan of the 3,000mm gauge Breitspurbahn which was never built.
Her broad-gauge sister, the 2,140mm gauge GWR Iron Duke class ran 80 mph in the late 1840s, for the matter.
The gigantic ''SS Great Eastern'' launched in 1858 impressed Verne enough to write a novel about it. It laid the first transatlantic telegraph cable in 1866 (and other cables thereafter) just because it was the largest ship available in the world. It had almost twice the displacement of a WWI battleship and it could sail as well as steam along.
Age of Steam saw also weird small arms designs which were far too modern and impractical for the time, as the first bullpup◊ rifle◊ in the world (1901) and the first semi-automatic◊ pistol (1893). In the same vein of firearm design, if most modern bolt-action rifles can trace their construction to the Mauser 98, but improved in detail over 100 years, there is a type of gun which reached perfection in 1875 and is still used in original form: the boxlock break-action rifle or shotgun. A modern double barrel gun is in no way technically different from the gun a Great White Hunter wielded.
The first fax machine appeared in 1888 - the golden age of steam. Jules Verne himself described it summarily in Propeller Island (1895) as "the device which wires writing as the telephone wires speech".
Since Doctor Steel was a Steam Punk musician and the founder of his fan site is a steampunk fashion designer, it isn't surprising to see steampunk fashion influences in the uniforms of the Army of Toy Soldiers.
The first programmable computer was designed by Charles Babbage using only mechanical processes. While the concept of the design was never fully realized in his lifetime, workable versions were built for much more simplified puposes, making the idea feasible. It wouldn't be until 120 years after his death that a complete and working version of it was build in 1991 using the plans for his Difference Engin No. 2 with materials and within the engineering tolerances of his time. It worked and there is interest in testing his design for the Analytical Engine he designed in 1837, which, if it works, would make it the oldest design for a Turing-complete computer.
There is a very serious problem in actually building the Analytical Engine compared to the Difference Engine No. 2: The Difference Engine was actually being built at one time (although almost all the original parts were destroyed after the effort collapsed) so complete construction plans were drawn up (including little details like the actual frameworks the parts were to be fitting into). The Analytical Engine never got past the conceptual design stage, so while all the mechanisms needed were designed, the details of how to actually put them together in a working configuration that will hold together were never done. The Difference Engine construction was simply producing a complex machine from blueprints, but the Analytical Engine will require first completing the design. (This may not be a bad thing, however. The Difference Engine was not what you'd call a good production design and was extremely difficult to debug. Almost all glitches in it caused the whole thing to lock up solidly, making it almost impossible to locate the problems and unbind the machine. The need to complete the Analytical Engine's design would, among other things, make it possible to modularize it so that portions could be tested and debugged in isolation, making it much easier to get and keep working.)
The Lippisch P.13a rocket-fueled interceptor, one of the many Wunderwaffe designed by Hitler's designers in the twilight of Nazi Germany. Because of Germany's awful supply situation, the designers anticipated that the complex chemicals needed to produce WWII rocket fuels would be in short supply. To solve this problem, they decided to use coal instead. Coal. In an aircraft. The rather clever propulsion consisted of a rotating drum of coal dust burned by natural gas jets, with the resulting fumes being mixed with clean air and re-ignited in a ramjet. Even more impressively, the engine worked, trialing successfully in Vienna. Sadly, WWII ended before the airframes could be completed and prototypes flown.
Like WWII, the American Civil War had its share of outrageous tech that never actually saw combat. Of course, everyone knows about the ironclads, the first steam powered iron-plated warships. But some you might not have heard of: A Confederate dentist named Dr. R. Finley Hunt drew up plans for a steam-powered airplane. And Union Colonel Edward Wellman Serrell designed a steam-powered helicopter called the Reconoiterer.