Here are ten(ish) of my favorite modules to extend the functionality of Drupal.

Views & CCK

Often regarded as two of the most powerful modules in Drupal. You can create new fields to define different content types with CCK and choose different ways of displaying them with Views. Views is the easiest way of creating and displaying customized collections of content on your website, and Views can easily output its result to a node, block or RSS feed.

SWF Tools

The easiest and most efficient way to embed Adobe Flash content onto your website. When paired with SWFObject 2 and my other blog post, this is the most compatible way to display videos on your website.

Backup and Migrate

This module makes maintaining your database a cinch. It can easily provide a way to download backups of your site for creating a local copy of your website. It also provides advanced options for restoring, migrating and scheduling regular backups. It makes doing one of those things that you should do, but don't always have the time to, a little easier.

Pathauto

This module provides the easiest way to automate the creation of URL aliases. Having good URLs aids in better site organization and can be SEO friendly. You can define custom rules that automate the task of defining the URL alias of a page.

Google Analytics

The simplest way to embed the Google Analytics tracking code onto your site. It also allows for advanced settings for tracking such as per-user or per-role tracking. Having a good analytics package is crucial to figuring out who has viewed your website and how they got there.

Wysiwyg

The best way to integrate a WYSIWYG editor to aid in the creation of content. This is a must have for anyone not familiar with HTML that wants to edit content on your site.

Mollom & Spam

As soon as you publish your first blog entry on your site, you will realize the need for spam protection. The spam module provides several filters to be able to block nonsense spam comments. The Mollom module integrates the Mollom web service to provide text analysis filter to further be able to block spam from being posted to your site.

Webform

Simplest way to create any type of custom form imaginable. Great for creating custom contact forms or surveys and has multiple options for exporting and viewing results.

Admin:hover

A simple module that provides administrative links in an unobtrusive way. Easiest and cleanest way of having necessary administrative links right at your fingertips.

Facebook-style Statuses

Great module to use on any social networking site. Includes constantly expanding features that enable user-to-user contact and integration. Don't be fooled by the name, FBSS is a lot more than just a mini-blogging platform.