Dictionaries for Mac is a macOS utility that integrates third party dictionaries (primarily Wiktionary-based) into the system as first-class citizens.
Built out of necessity, scratching my own itch, back when Apple’s built-in dictionaries did not support many languages (including my native Czech), it has evolved into a larger project with pronunciation support and other features.
Later expanded to Kindles too.
Lots and lots of interesting reverse engineering through the years, and a fun project to work on.