Downcrawl

Publisher's blurb:

Downcrawl is a system to help gamemasters run randomly generated, open-ended adventures in a weird and fantastical underworld called The Deep, Deep Down: a place so far from the surface that the sun and sky are only legends, and so vast that no bounds can be placed on its dimensions or contents.

Compatible with any fantasy roleplaying system, Downcrawl contains rules for taking dangerous journeys through unmapped places, tables of fungi with curious effects, procedures for GMs to generate strange new peoples, places, and encounters, and tips for running a satisfying, spontaneous campaign when your players might explore in any of six directions. Oh, and huge intelligent spiders who eat faces. Watch out for those.