paradisebunny
Explorer
Just to add to system neutral adventures: there is the dark of hot springs island, a dark tropical island hexcrawl sandbox and the great collection of one page Trilemma adventures
Ravensrook is a great system-neutral city setting book. It very much calls to mind dangerous cities of fiction such as Sanctuary and Lankhmar. I've personally used it as a stand-in for the Free City of Greyhawk when I wanted a city with a slightly more grungy feel and also as a city "core" for the City League in Pelinore.
It doesn't appear to be legally available online, and the few physical copies I could find are far too expensive.I could swear that it was available as a pdf a few years back but I can't find it on drivethrurpg if that was the case.