From Dungeons of Dredmor Wiki

The Zorkmid is the standard unit of currency used in Dungeons of Dredmor. Zorkmids have a random chance to drop off of Monsters when one is killed, or when the character or a Monster are affected by a Midas attack, or they can simply be found randomly lying around on the dungeon floor.

Zorkmids can be used to buy items from Brax or from various vending machines around the dungeon.

[edit] Trivia

• Zorkmids were originally used as currency in the world of Zork, but have become a de facto standard currency for many video games, including an older Roguelike-game, NetHack.