Dieselpunk is like Steampunk, but instead of the Industrial Revolution, we got both World Wars and period between them. Moreover, Dieselpunk is used far less in popular media than its brass and goggles counterpart.
If steampunk is all about brass, steam, blimps and clockwork robots, then dieselpunk is steel, gas masks, dark smoke, oil stains, tanks and dreadnoughts (of the naval kind, and the walking boxes kind).
Whereas Steampunk borrows the fashions and aesthetics of the Victorians, Dieselpunk's primary aesthetic is Art Deco. Anything not related to machines will be very ritzy and flashy, jazz will be the dominant form of music and futurism will be the dominant form of architecture. Expect a lot of pressed and stamped metal. Thin, gilded veneers concealing oily machines (both literally and allegorically). Decopunk describes a setting which has all the flashy and the futuristic aesthetics found in parts of Dieselpunk, but none of the grime and realism that characterises most Dieselpunk works.
The overall technology level hovers around 1935: we got machine guns, cars, piston planes (and jets, but they're bleeding-edge tech that isn't found outside the military or corporations), radios and black-and-white TV sets. Electronics are something of a gray area: they exist, but they can't be very advanced. You can expect radio, radars, alarms, simple encrypting/decrypting machines and so on, but no portable phones, detectors, transistors and computers (barring occasional ENIAC-styled vacuum tube monstrosities that take up at least an entire room). In a nutshell, if it looks like your grand-granddad would like it, that's steampunk, if it looks like it belongs to your grandpa, that's dieselpunk, and if it looks like your dad used it, that's early Atompunk, assuming you're about 40 (which you are, unless you're a newfag).
Also worth mentioning that there is a significant difference in mood: steampunk stories often lean on the romantical side of things, adventuring with the science of the future and all that. Dieselpunk, on the other hand, gets grimdarker with industrialized warfare, totalitarianism, and nihilism. Even without the shadow of "The Great War/s" looming over the setting, you have the social unrest of the Roaring 20's, the Great Depression, Gangland, and ruined continental Europe rebuilding in the background to temper your optimism. Steampunk is "The Time Machine" and "20,000 Leagues Under The Sea", Dieselpunk is "All Quiet On The Western Front" and "Catch-22".
Much like in popular media, Dieselpunk only has a few tabletop titles to its oil-smeared name.
- Crimson Skies is a board game from '98 which focused on 2-player aerial dog fighting with nice cardboard models and while it was decent, copies now are hard to find, unfortunately. The IP was since adapted into a video game, both in 2000 and 2003. Perhaps surprisingly, both titles are highly reviewed.
- Age of Steel is an RPG released in current year+3 which promises a D6-system mixed with Dieselpunk and the Cthulhu Mythos. However, the core rulebook is only a poultry 123-pages.
- Scythe was released in 2016 and is an alternate retelling of the 1920s featuring deadly walkers and serfdom. The strategy board game has garnered lots of awards and is getting several expansions as well as a digital release.
- Dust Tactics is a board game from 2010 featuring an alternate take on WW2 where alien tech made walkers, minigun-wielding gorillas, and tesla cannons possible. The game is for 2-4 players and has models representing American, German, and Soviet units. The game since tried to expand into a full wargame, but it failed to grab an audience. The game has since been unlisted from FFG's website.
- Imperial technology (and, by extension, Chaos and Ork tech, since they like to loot their tech) is the embodiment of this. Rule of thumb: if it doesn't look like it came out of Renaissance Fair or cheap 80'ies sci-fi show (or both), it's probably a Dieselpunk design. The Imperial Guard is literally runs on it, special mention goes to Kriegers, Armageddoners, Mordians, Valhallans and the entirety of the Commissariat.
- Generally, every Russian or Russian-equivalent faction in a sufficiently advanced setting is this; see Khador, Red Bloc; good vidya examples are Soviets from Red Alert and China from C&C:Generals.
- In /v/, examples are Fallout (albeit if you're inclined to split hairs, that's Atompunk (50'es to 70'es)), first two Bioshocks (Infinite is more steampunky), Dishonored and Wolfenstein (from "Return to" and onwards). Primorida also covers this aesthetic, though mixed with some very mother-of-pearl sculpting and hiding beneath the post-apocalyptic surface. The Hungarian shmup Sine Mora EX also deserves mention for its many stylish designs of planes, tanks, and warships. Although, it also has furries committing genocide. Make of that what you will.
- Dieselpunk has also reared its head in Hollywood. The movies The Rocketeer and Sky Captain and the World Tomorrow are both aesthetically accurate but suffered mediocre reviews.
- For weeaboos goes the non-alchemy bits of Fullmetal Alchemist, half of Miyazaki's creative output, Avatar's Fire Nation, most of The Legend of Korra, and the 2001 animated (pseudo-remake) Metropolis.
- A new RTS game called Iron Harvest. Literally a dieselpunk WW2, in a world where we made humanoid mechsuits instead of tanks. Based off of the 1920+ universe.