With ever increasing popularity of WordPress, Gravatars have become quite popular. It is very easy to integrate them into your application as they provide a very easy to use API.

Use Gravatars in Your Application

<?php
/******************
*@email - Email address to show gravatar for
*@size - size of gravatar
*@default - URL of default gravatar to use
*@rating - rating of Gravatar(G, PG, R, X)
*/
function show_gravatar($email, $size, $default, $rating)
{
  echo '<img src="http://www.gravatar.com/avatar.php?gravatar_id='.md5($email).
		'&default='.$default.'&size='.$size.'&rating='.$rating.'" width="'.$size.'px" 
		height="'.$size.'px" />';
}