Ultra-simple example of using a filter to write a basic WordPress plugin

simple_sign_up_message.php

<?php
/*
Plugin Name: Sign Up Message
Description: A basic plugin for illustration/learning purposes.  Thanks subscribers at the end of a post.  Not logged in users get asked to sign up. 
Version: 0.1
Author URI: http://www.digitalblockarea.com
*/

function simple_sign_up_message($content){

  if ( is_user_logged_in() ) {
    $current_user = wp_get_current_user();
    $message = 'Hi, ' . $current_user->user_firstname . '.';
    $message .= ' Thanks for reading!';
  }

  else {
    $message = 'Hi, and thanks for reading! ';
    $message .= '<a href="'. wp_registration_url() .'">Subscribe to get cool stuff!</a>';
  }

  return $content . $message;
}

add_filter('the_content', 'simple_sign_up_message');