As many of you already know, Drupal is one of the best and most popular content management systems for web sites.  Drupal has an excellent community of developers and contributors.  As a result, there are a ton of modules available.  The following modules are my favorites and the ones I use most often.

Content Construction Kit (CCK)

CCK is by far my favorite module.  I can't imagine a site being built in Drupal without it.  CCK allows you to create new content node types, as well as add additional fields to an existing content type.  There are a number of additional modules you can install for just about any sort of form field you can think of.  This module is so important to Drupal, they are going to include it in the core modules for the 7.x release.

Visit the CCK project page

Views

Views is another very useful module and is a must have for any Drupal installation.  With Views, you can build lists from any data source in Drupal and customize the look and feel of the data.  The way it works is similar to designing a query in a database query designer application, but it is much simpler and offers far more flexibility.  I think Views should join CCK as a core module.

Visit the Views project page

Administration Menu

The administration menu module creates a dropdown menu at the top of the page when logged in as an administrator.  This is very useful for quickly navigating to the various admin areas of the website.

Visit the Administration Menu project page

Nodewords

The nodewords modules allows  the editing of meta tags for any content created in Drupal.  This is very useful for SEO purposes to have precise control over the meta tags and content of those tags appearing in your website code.

Visit the Nodewords project page

Global Redirect and Pathauto

These two modules work well together to aid in rewriting the url to a more human friendly version.  Global Redirect will redirect any duplicate paths to the correct alias.  Pathauto allows you to manage the alias generated for each page of content.

Visit the Global Redirect project page

Visit the Pathauto project page

Rules

This module allows the administrator to setup rules, conditions, and actions.  Using the rules module, you can modify the workflow of your website.  Say you want a user to be directed to a specific page when they login if they have a certain role.  This module can do that, and much more.

Visit the Rules project page

Google Analytics

Every site needs to have some sort of analytics tracking.  Analytics are essential for improving your website by studying trends.  One of the best packages out there is Google Analytics.  Google provides a very easy to use interface for viewing website statistics and trends.  The best part about it?  It's free.

Visit the Google Analytics project page

Webform

The webform module is useful for creating any sort of form within node content without knowing how to program or create your own module.  I have found it especially useful for custom contact forms.

Visit the Webform project page

Captcha and reCaptcha

These two modules are useful for stopping spam submissions in your website forms.  The reCaptcha module presents the user with two words to enter from a distorted image.  I have found this one to be the most effective and easiest to implement captcha system.  The captcha challenge can be applied to any form registered with Drupal.

Visit the Captcha project page

Visit the reCaptcha project page

Validation API

This module allows the administrator to define custom validations for any form field on your website.  I have found this very useful for CCK Field validation beyond the built-in validation.

Visit the Validation API project page