Ensures only one user at a time can be logged into WordPress, ie, 2 people can't login using the same account.

single-user-loggin.php

<?php
/*
Plugin name: Single user login
Plugin URI: 
Description:
Author: Ben May
Author URI: 
Version: 0.1
*/

if( !class_exists( 'WPSingleUserLoggin' ) )
{
  	class WPSingleUserLoggin
	{
		private $session_id; 
	
		function __construct()
		{
			if ( ! session_id() )
			    session_start();
	
			$this->session_id = session_id();
	
			add_action( 'init', array( $this, 'init' ) );
			add_action( 'wp_login', array( $this, 'wp_login' ), 10, 2 );
		}
	
		function init()
		{
			if( ! is_user_logged_in() )
				return;
				
			$stored_sess_id = get_user_meta( get_current_user_id(), '_wp_single_user_hash', true );
			
			if( $stored_sess_id != $this->session_id )
			{
				wp_logout(); 
				wp_redirect( wp_login_url() );
				exit;
			}
		}
		function wp_login( $user_login, $user )
		{
			update_user_meta( $user->ID, '_wp_single_user_hash', $this->session_id );
			return;
		}
	}
	new WPSingleUserLoggin();
}