This is a custom update checker library for WordPress plugins. It lets you add automatic update notifications and one-click upgrades to your commercial and private plugins. All you need to do is put your plugin details in a JSON file, place the file on your server, and pass the URL to the library. The library periodically checks the URL to see if there's a new version available and displays an update notification to the user if necessary.
From the users' perspective, it works just like with plugins hosted on WordPress.org. The update checker uses the default plugin upgrade UI that will already be familiar to most WordPress users.
See this blog post for more information and usage instructions.