If you ask this question often, and you have a spare web server with PHP running somewhere, here's a little tidbit to make your life much easier.

<?php
header('Content-Type: text/plain');

echo $_SERVER['REMOTE_ADDR'] ."\n";

if(isset($_GET['hostname']) or isset($_GET['all'])) {
    echo gethostbyaddr($_SERVER['REMOTE_ADDR']) ."\n";
}

if(isset($_GET['useragent']) or isset($_GET['all'])) {
    echo $_SERVER['HTTP_USER_AGENT'];
}

?>

Just make a vhost with the name ip.yourdomain.com and ask people to copy paste or screenshot this.

You can also send a link to http://ip.yourdomain.com/?useragent to get their useragent string, or ?hostname to find their reverse DNS name. ?all will give you everything in one package.

I can't count how many times I've been to slow, ad-ridden sites that load 50 megabytes of images and videos to show me my public IP. It's embarrasing to send people these links. This one I made so it could be easily scriptable and FAST.

If you have any suggested improvements, please let me know.