Expedition to the Barrier Peaks
Expedition to the Barrier Peaks (module code: S3) is a Dungeons & Dragons module created by Gary Gygax for the Greyhawk setting in 1976. It is one of the most infamous of 1e modules, as it was the first ever "science fantasy" (fantasy/sci-fi crossover) module created for and officially supported by D&D.
The plot is simple; the party has been recruited to travel to the Barrier Peaks from the Grand Duchy of Geoff, which has recently been attacked by a plague of bizarre and unearthly monsters that not even the most learned sages can identify. The source of the monsters is a strange dungeon, guarded by doors of impenetrable metal, that has been discovered in the Barrier Peaks. The party must enter the dungeon and put a stop to this tide of abominations, and if possible find out what the hell is going on.
Unbeknownst to them, the strange "dungeon" is actually an ancient crashed spaceship, still full of technological items like power armor and laser pistols, dangerously malfunctioning robots, and alien horrors from beyond the stars. A combination of earthquakes and computer malfunctions has finally exposed part of the spaceship to the world above, and caused worker robots to begin off-loading the cargo of alien flora and fauna. The party must try and make its way through the spaceship, avoiding or overcoming all manner of threats and hopefully without ending up eaten, irradiated to death, consumed by russet mold and their remains fashioned into a vegepgymy, or killed by accident when fooling around with one of the advanced weapons or medical elixirs to be found within.
One of the things that bothered players for many years after the module was released was simple; where the hell did the spaceship come from? Eventually, in the final print issue of Dragon Magazine, #359, Wizards of the Coast gave the answer; as obliquely hinted in the adventure itself (or outright stated in some versions), the "dungeon" is in fact a module broken off from the titular super-sized colony ship of Metamorphosis Alpha, swept through a planar rift and deposited on Oerth.
Although its legacy is contentious - many players regard it as "too silly" or "breaking the themes of D&D", the module has had a small impact on game rules. Dragon Magazine #410 included an Bazaar of the Bizarre article entitled "Thingamajigs of the Barrier Peaks", which adapted power armor, blaster rifles, chainswords, laser pistols, needler pistols, confusion rays, blue communicators, jetpacks, security cards, motorized sleds, singularity grenades and sleep grenades to magical items for Dungeons & Dragons 4th edition. Meanwhile, in 5th edition, the 4e rules for PCs handling futuristic gear were slightly adapted and included alongside the rules for firearms in the DMG, with the table of sample Firearms including lasers and antimatter rifles. Amusingly, the very same page included a cut-down version of the artwork from the 4e article; a female adventurer in a jetpack and power armor blasting an otyugh with a laser pistol from midair as it tries to grab her. 'The Lost Laboratory of Kwalish', a digital-only adventure for 5th Edition (written as a charity piece for Extra Life), depicts another expedition into the Barrier Peaks, specifically looking for the long-missing titular arcane researcher and his knowledge for various reasons. This not only sees an official published adventure for Fith Edition that can actually use those DMG laser guns without it feeling off and gives us new stats for the Power Armor and it is both suitably complicated and powerful.
Pathfinder, meanwhile, based an entire region of Golarion on the module in the form of Numeria (which borrows from Conan the Barbarian). It's a backwater place where an ancient gigantic starship crashed, ensuring that barbarians and sci-fi mad science exist side by side there. One splatbook, the Technology Guide, and the Iron Gods Adventure Path is set entirely within the region, with part one of the latter (Fires of Creation) being a more direct homage to the original module.