I made a zine of random tables for generating weird underground caverns, and I wrote a simple RPG system to go along with it.
The Eternal Caverns of Urk produces stuff like this:
"This cavern has porous red spires, rainbow geometric trees and black moss. It has no water source and the air is sweltering and dry.
This cavern is occupied by obsequious tattooed hairless cat folk with a cultural disposition for baroque fashion. They live in colorful patchwork yurts and are seeking to recite poetry and learn new poems (and be very critical of them).
Notable wildlife in this cavern includes the shimmering fungal bear and the whispering shadow bear."
Tunnel Goons is included in Caverns of Urk but is also available for free separately. I think it's quite hackable and hope to build some other games on top of it.