Small is beautiful.
Corey Ryan Walden's twelve page The Cliff-Lair of Heeter is small, location-based adventure written in a system-neutral way that demands to be dropped into an existing campaign at the first appropriate moment. As the title implies, this adventure details the working home of the curious entity named Heeter, who is of a species apart from those traditionally found in Old School rules. Part tyrant, part child, Heeter is surrounded by "manlings" that recall the Oompa Loompa's of Willie Wonka. And he has a weird, fanged cat-thing to boot.
The cliff-lair could be used a throw-away, low-level encounter, but that would be a waste of a good, characterful NPC. Heeter seems like a natural fit to be a source of adventure lore or a quest-giver. One could trim some of the friendlier eccentricities away from Heeter and his home to create a darker, more dangerous encounter with an alien thing, whose desires are quite unintelligible to the party.
It should be noted that this work is vague on game mechanics, so be prepared to fill in those blanks with specifics from your preferred ruleset.
At under $2, this PDF provides a solid value of RPG content and is a solid resource for those game masters who enjoy cobbling together adventures from various bits.