TROIKA! (Science Fantasy RPG) is free if you join a mailing list. The 144-page bookmarked PDF is rules + an adventure. Portal, labyrinth or barge lets you encounter goblins, dragons, living dead, etc

The title is heavily edited for the main ideas of the game. Here is the blurb from the PDF:

...what you have here is Troika!, a science-fantasy RPG in which players travel by eldritch portal and non-euclidean labyrinth and golden-sailed barge between the uncountable crystal Spheres strung delicately across the Humpbacked Sky. What you encounter on those Spheres and in those liminal places is anybody’s guess – I wouldn’t presume to tell you, though inside this book you will find people and artefacts from these worlds which will suggest the shape of things. The adventure and wonder are in the gaps; your game is defined by the ways in which you fill them.