node_class

Node Class is a simple module that allows users to add custom CSS classes to any node through the node/add interface.




For Drupal 6 only
Add the following PHP snippet to your node.tpl.php

<?php print node_class($node) ?>

So your new node.tpl.php

<div id="node-<?php print $node->nid; ?>" class="<?php print node_class($node) ?> node<?php if ($sticky) { print ' sticky'; } ?><?php if (!$status) { print ' node-unpublished'; } ?>">

Tutorial: Node class (Video tutorial)

Related Module:
Node type class: This module adds class per content type.