Code is poetry. An implementation of a plugin class structure that allows you to access a plugin class instance without using singletons and without storing a reference to the instance in the global namespace.

myplugin.php

<?php
/*
Plugin Name: My Plugin
*/

/**
 * Main plugin class
 */
class MyPlugin {
	
	/**
	 * Constructor
	 */
	public function __construct() {
		
		// Only add actions and filters here
		add_action( 'plugins_loaded', array( $this, 'after_setup' ) );
	}
	
	/**
	 * Fires when My Plugin is fully loaded
	 */
	public function after_setup() {
		
		/**
		 * Fires when My Plugin is fully loaded, but no real actions have been performed yet
		 * At this point, all required files have been included and all actions and filters have been added
		 *
		 * @param MyPlugin $myplugin_instance Main My Plugin class instance
		 */
		do_action( 'myplugin/loaded', $this );
	}
	
}

new MyPlugin();

usage-example.php

<?php
/*
Plugin Name: Another Plugin
*/

// Simplified usage example for the setup

class AnotherPlugin {
	
	public $myplugin;
	
	public function __construct() {
	
		// Hook into My Plugin setup callback
		add_action( 'myplugin/loaded', array( $this, 'myplugin_loaded' ) );
	}
	
	public function myplugin_loaded( $myplugin_instance ) {
		
		// Store reference to My Plugin's main class instance for usage throughout Another Plugin
		$this->myplugin = $myplugin_instance;
	}
	
}