Add first and last classes to your WordPress loop without using JavaScript

gistfile1.php

<?php
/**
 * Display the classes for the post div and automatically mark the first and last posts
 * 
 * @param string|array $class One or more classes to add to the class list.
 * @param int|WP_Post $post_id Optional. Post ID or post object.
 */
function fjarrett_post_class( $class = '', $post_id = null ) {
	global $wp_query;

	$class = ! empty( $class ) ? sprintf( '%s ', trim( $class ) ) : '';

	if ( 0 === $wp_query->current_post ) {
		$class .= 'first';
	} elseif ( ( $wp_query->current_post + 1 ) === $wp_query->post_count ) {
		$class .= 'last';
	}

	echo get_post_class( $class, $post_id );
}