RPGaDAY2024 29. "Awesome app"
The answer to this prompt is The Crawler's Companion (for Dungeon Crawl Classics RPG and Mutant Crawl Classics RPG) from Purple Sorcerer Games. I highly recommend both the app and the HTML version.
https://www.purplesorcerer.com/crawler.php
https://www.purplesorcerer.com/index.php