I usually sublimate my imaginary worlds into role-playing game settings - sadly, I've only had the opportunity to run a couple of games in them...
For fantasy, we have the world of Aathe. Untold millennia ago, Aathe was a world of high science, accomplishing wonders with almost monotonous regularity. Unfortunately, they eventually developed a true "ultimate weapon" - a bomb that dissolves gluons, disassociating matter at the quark level. Equally unfortunately, the masters of these weapons thought of them as just bigger, better bombs - and used them. That event is known to folklore as the Suicide War; folklorists speak of the Old World as a place where the men were gods.
The bombs tore holes in reality itself; when the handful of survivors emerged from their shelters, they discovered that their world had been replaced by a more pastoral one, with elves, dwarves, orcs, and all the other races of fantasy. Magic worked here, and ten of the survivors were elevated to actual godhead (although they're known as the Nine Gods, because worship of the tenth, Technos the Uncaring, God of the Machine, is officially forbidden - mythology holds that He was responsible for the Suicide War). Thousands of years later, the great Aathean Empire arose, girdling and civilizing the entire planet. It was quite inclusive - at the Empire's height, its Royal Council included two dragons and a vampire archmage. Sadly, it eventually fell; its decline began with the Chaos Wars, when the mage Loran the Mad sought to replace Pasha Warlady as the god of magic and warfare, and culminated in the reign of Emperor Klaren VII, the Bloodletter, when he tried to destroy the leaders of the elven, dwarven, and halfling nations in an event recorded as the Midsummer Massacre.
Today, the adventurers live in a world not unlike the basic D&D "Points of Light" setting - the smaller continent, Aath, is home to the Second Aathean Empire, while the greater continent, Cerga, is a place of petty kingdoms, lands that haven't seen humans in five thousand years, and a vast eastern area of which the Empire knows nothing - it's employing the adventurers to explore and report. There are also the Reality Gates, holes in space/time left over from the Suicide War - some are permanent, some are cyclical, and some open once, close, and never reappear, usually after depositing someone (or something) in Aathe.
The other world I've created is for superhero RPGs; the history of costumed adventurers starts in the 1920s with the Black Cat, who used his superhuman agility and strength to fight mobsters, and really exploded in WWII, when the Allies sought parahumans to staff the Blue and White Star Squadrons (the Blue Stars fought in Europe, against the Ubermenschen, and the White Stars were in the Pacific theater, opposing the True Children of the Rising Sun). (There was also a Soviet alliance group, the Red Star Squadron, who fought Nazis and the Ubermenschen in Russia.) After the war, the Blue and White Stars formed the basis for both the official American group, the All-Stars, and the UN group, the Protectors of Terra. The Red Stars became the People's Heroes of the Soviet Union. Most of the Ubermenschen were convicted of war crimes and either imprisoned or executed; a few recanted their Nazi service, and were pressed into service by the new German goverment. The Rising Sun, humiliated by their Emperor's surrender, committed mass seppuku. Today, all that is long in the past; there are parahumans all over the planet, including a number of supergroups. I've worked out most of the legal changes that would be required to make masked vigilantes work (although the usefulness of the Parahuman Activities Bureau rests on the fact that its top people are all monitored by telepaths, to make sure nobody compromises the secure database of registered heroes - they are required to sign releases authorizing this monitoring, and the telepaths are restricted by law to release only information pertaining to the Bureau's security, not the private lives of the individuals concerned).
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.