plugin-update-checker

A custom update checker for WordPress plugins. Useful if you don't want to host your project in the official WP repository, but would still like it to support automatic updates. Despite the name, it also works with themes. Maintained by YahnisElsts on GitHub, where it has earned 2,545 stars from the community.

It's actively developed around wordpress, wordpress-development, and is a solid reference for anyone building with these tools.