This class autoloader function eliminates the need to include_once every class file in your WordPress plugin. It adheres to WordPress file naming conventions, so instantiating a class named MyClass would attempt to include_once a file called inc/class-myclass.php.

plugin-class-autoloader.php

<?php
/**
 * Plugin Name: My Plugin
 * Description: A sample plugin with class autoloader, for object-oriented programming.
 * Version: 0.1
 * Plugin URI: http://shaunscovil.com/
 * Author: Shaun Scovil
 * Author URI: http://shaunscovil.com/
 * License: GPL2
 */

function my_class_autoloader( $classname ) {
    $classfile = sprintf( '%sinc/class-%s.php',
        plugin_dir_path( __FILE__ ),
        str_replace( '_', '-', strtolower( $classname ) )
    );
    if ( file_exists( $classfile ) ) include_once( $classfile );
}
spl_autoload_register( 'my_class_autoloader' );