One of Drupal's great strengths is the ease with which new functionality can be added in the form of contributed modules. The clear and well-defined hook system (http://drupaldocs.org/api/head/group/hooks) allows modules to interact with all of the Drupal subsystems, including the user, menu, taxonomy, filtering, and node-handling systems. As a result, more than 350 modules have been contributed to the Concurrent Versions System (CVS) repository (http://cvs.drupal.org/viewcvs/contributions/) at http://drupal.org/project/Modules.

The contributions repository is not only large, but it is also very diverse in its offerings. Some of the modules are for specific Drupal versions; some are well maintained, but some are not. Some are easy to install; some require patches to the core Drupal code. Many offer similar or duplicated functionality. This diversity is a great strength of Drupal, but it also means that you need to know which modules are best for any given situation and task.

This chapter will cover the installation, configuration, and use of a number of the best and most popular Drupal modules. The selection covered ranges from making input easier with WYSIWYG editors, to protecting your site from comment spam, to letting site users organize themselves into groups that share interests. These modules, in conjunction with the core modules delivered with Drupal, will provide you with a broad set of tools that you can apply to a diverse array of web sites.

Getting Drupal Modules

You can find a list of the available modules for the current release at Drupal.org:
http://drupal.org/project/Modules
Each module has a download link to an automatically generated archive file, which is updated with the latest changes from Drupal.org's CVS repository on a regular basis. First, I'll give you an overview of the modules covered in this chapter, and then I'll explain the general installation process.

Introducing Some Useful Modules

Some of the most useful modules are those that help with creating content on your site. These include the TinyMCE module (WYSIWYG editor) and the Image and Image Assist modules (for image galleries and in-line images in posts).

The Flexinode module allows you to define your own data types (node types), complete with fields of varying types. The data collected with your defined node types can be shown in table form with sortable columns to facilitate searching.

The Event and Location modules can be used alone or in tandem to attach time and place information to any node type. Combined with the Flexinode module, the Event and Location modules allow you to create custom events calendars that not only track events, but also offer location information based on zip codes and geocoding, as well as deep linking to mapping services.

Organic groups are a way to encourage members of your site to organize into smaller working groups to share resources and have a space for online collaboration. The Organic Groups module introduces node-level permissions so that groups can have private content not visible to those who aren't members of the group. Group photo albums and RSS feeds for each group are among the other features available.

Protecting your site from abuse by spammers is the focus of the Spam module. Based on powerful Bayesian filters that continuously learn from the spam content that is submitted to your site, this module may be the only thing that keeps spammers from turning your site into an online billboard for drugs and sex products.

The Devel and Database Administration modules offer convenient tools for maintaining your database and developing Drupal code. They give you a window into the inner workings of Drupal, exposing not only the contents of your Drupal database, but also the actual SQL queries that are used to build each page. Turn to these modules to help solve database-related problems.