One of the most common questions we get from new Drupal users is, "Which modules do people normally use?"
That's a big question, with over 20,000 modules, but some are far more popular than others.
Webchick, one of the Drupal core developers, has used the statistics available on Drupal.org to make a list of the most popular Drupal 7 modules. This is different from the public stats on Drupal.org module pages, which show the popularity of modules across all versions.
Here's an introduction to the 20 most popular Drupal 7 modules:
#1. Chaos tool suite
Chaos tools suite is the most popular Drupal 7 module but it's not one you're likely to interact with directly.
Chaos tools (or CTools) as it's often known, is a set of APIs that assist other modules. In this top 20 list alone, CTools is used by Views, Views Slideshow, Webform. It's also required by popular layout modules such as Panels and Display Suite.
Views the most popular module when you count all Drupal versions. Why? Because Views is the way to organize content in Drupal. It's so essential to so many sites that it will be part of the Drupal 8 core.
Here's a presentation I gave explaining what Drupal can and can't do without the help of Views:
Token allows you to create re-usable patterns across your site. For example, the Pathauto module (#5 in this list) uses Token to create URL patterns. Here are some examples of Token in action:
#4. Libraries API
Libraries API is similar to Chaos Tools in that it is a foundational module. It's an essential part of the WYSIWYG and Colorbox module and powers several Views Slideshow features.
Libraries API works to connect Drupal to external scripts so if a module asks you to install Libraries API you'll probably need to download and install a 3rd party script also.
Pathauto creates clean, keyword rich URLs in Drupal.
Pathauto, together with the core Path module, is the difference between having URLs like this /?q=node/3 or this /node/3 on your Drupal site, and having URLs like this /about-us/. Pathauto combines with Token (#3 in this list) to allow to create default patterns for your URLs. Here's a video introduction: