If you hang around older roleplayers you hear about three truly great and classic settings: Glorantha of Runequest and Heroquest as well as several board games; Tekumel of Empire of the Petal Throne, miniatures rules and a couple other sets of miniature rules; and Jorune.
I first heard of Skyrealms of Jorune from ads in Dragon Magazine. It was the product of an English class assignment and an early Metamorphosis Alpha campaign. I never had the first edition but apparently it was much more gonzo than the later editions. Given the nature of the world in the second and third, which I do own, more gonzo is saying something.
The world of Jorune was originally a colony planet of humanity. A preserve for humanity had been negotiated with the native race, the Shanthas. When news of a apocalyptic war on Earth reached the colony humans burst out of their preserves in a grab for more space. In response the seemingly primitive Shanthas destroyed the entire colony. Humanity was so scattered that one human geneticist uplifted several Earth animals to preserve the races of earth.
Now, several hundred years later humanity, diverted into several species including one that can use isho the native energy of Jorune, and about a dozen other intelligent species, both from Earth and elsewhere, compete for the planet. Starting characters are Tauther, potential citizens of the major human kingdom. Tauther become citizens by having citizens inscribe Earth metal tablets with their endorsement. This provides a great framework for a very old school picaresque campaign.
The second edition was a boxed set. Initially it had a custom system that was much revised in an insert to a Runequest like system. Isho wielding provides magic while lost Earth Tec from the early colony period provides super science. The result is a unique science-fantasy setting that seems parts Gamma World, Tekumel, and standard D&D. A few supplements were produced. Two covered regions of Jorune. The rarest, Earth Tec Jorune, would be a great addition to science fantasy campaigns such as The Metal Earth or Planet Algol. About this time there was also a DOS game, Alien Logic.
A later, third edition, was from Chessex and was poorly received due to editing errors. This is sad given it was a labor of love from a member of the Jorune community. Sadly, Jorune never seemed to develop the community that Glorantha and Tekumel have. There is an excellent website with lots of additional material and conversations to about every game under the sun but that seems to be about it. The core properties seem to be lost to the dustbin of unsupported games.
One last note to the Jorune. The Skyrealms of the title were free floating islands created when crystaline magma cooled in pockets near the surface. The same energy field that created isho powers also caused the crystal to levitate breaking off a floating island. The second edition boxed set has an adventure set in (and named after) The SkyRealm Kolovisondra. Finding the explanation with diagrams for a skyrealm could be a great addition to many games.