In my opinion, there are two modules that illustrate the power of Drupal better than any others.
The first module is Views, which most people become familiar with as soon as they learn Drupal. The other is Rules.
Rules is the basis for almost any workflow that's set up in Drupal. You probably need Rules, whether you're running an intranet, a social network, an e-commerce site, or any site that needs "First this, then that" commands.
In this tutorial, we're going to show you how to use Rules to automatically update a node. We're going to make a record of the last person to see this node: