Add Custom Class To Last Post In A Loop

// Add a class to the last post in a loop
function last_post_class($classes){
	global $wp_query;
	if(($wp_query->current_post+1) == $wp_query->post_count) $classes[] = 'last';
	return $classes;
}
add_filter('post_class', 'last_post_class');

Source