There are plenty of Drupal Modules that you can install for your site as add-ons. What we liked best about Drupal modules is that it already comes pre-shipped with several modules. So a standard distribution contains several useful modules that you can simple point, click and enable.
Some examples of such pre-shipped modules are: archive that displays a calendar to navigate old content, book which allows users to collaboratively author a book, forums for threaded discussions about general topics, paths that enable renaming URLs for search engines, polls to capture votes on different topics in form of multiple choice questions, and much more.
You can also download many user contributed modules from Drupal site that are not pre-shipped in standard distribution. You can see full list of modules at: http://drupal.org/project/Modules. Here are some modules that we found interesting:
* API: This is an implementation of a subset of Doxygen documentation generator specification, tuned to produce output that best benefits Drupal code base. This module was designed to produce Drupal developer documentation available at drupaldocs.org. http://drupal.org/node/6018
* Banner: The banner.module allows you to display ads on your Drupal website. It randomly displays banners, and automatically tracks how many times each is displayed and clicked. Users of your website can be given ownership of banners, and be allowed to modify certain settings and view statistics. Supports numerous graphic formats, flash animations, and text. http://drupal.org/project/banner
* Chatbox: This is a simple chatbox module. It allows your site's visitors to chat in an HTML interface. http://drupal.org/project/chatbox
* Database Administration: The dba module provides Drupal administrators with direct access to their Drupal database tables from within standard Drupal user interface. It is possible to execute scripts to create and alter tables, to backup one or more tables, to view/edit/delete data within tables, and to emtpy or drop tables. If using MySQL, it is also possible to check and repair tables. http://drupal.org/project/dba
* E-Commerce: A collection of modules used to sell goods and/or services. Some features include:
o Subscriptions and recurring payments o Sell file downloads, shippable items, and even collections of various products as a single item o Inventory management enabled on a per-product basis o Payment and shipping components are 'pluggable'. The system can use Paypal, authorize.net or you can roll your own. o Invoice generation and shipping notifications for tangible products o Shopping cart and product 'look and feel' are themeable o Transaction reports and sales summaries o Transaction and payment workflow o Customers can review their order history. o Dynamically adjust item prices for a given group of users o Run an auction site with contributed auction module.
* Event: This is a simple module to keep track of events and show them to users. http://drupal.org/project/event
* FCKeditor for Drupal: This module allows Drupal to replace textarea fields with FCKeditor. This HTML text editor brings to web many of powerful functionalities of known desktop editors like Word. It's really lightweight and doesn't require any kind of installation on client computer. NOTE: FCKeditor for Drupal relies on an external library called fckeditor. For further information please refer to: http://www.fckeditor.net http://drupal.org/node/16118