External Update Manager --

"A drop-in library for WordPress themes or plugins to manage updates."

self-hosted...can't be submitted to official WordPress repository...non-GPL licensed...custom-made...commercial...etc.

Requirements

Grab the class-external-update-manager.php file and place it somewhere inside the theme or plugin directory
Add a require_once call in the theme's functions.php or in the plugin's main php file referencing the class file
Run the EUM_Handler with the full path of the theme or plugin and the update URL to check for the latest version available

require_once 'class-external-update-manager.php';
EUM_Handler::run( __FILE__, '<UPDATE URL>' );