"WordPress is a state-of-the-art publishing platform with a focus on aesthetics, web standards, and usability. WordPress is both free and priceless at the same time." - WordPress.org

"b2evolution ... includes all the features of traditional blog tools, and extends them with evolved features such as file & photo management, advanced skinning, multiple blogs support as well as detailed user permissions...
Not to mention third party plug-ins!

Plus, it's free, open-source (GPL), it runs on virtually any webserver featuring PHP+MySQL and it's available in many languages!" - b2evolution.net

"Since its creation in 2000, phpBB™ has become the most widely used Open Source forum solution. Like its predecessors, phpBB™ 3.0 “Olympus” has an easy to use administration panel and a user friendly installation process, which allows you to have a forum set up in minutes.

With a large and highly customisable set of key features coupled with a standards compliant interface, phpBB™ will grow with, and complement your website. With millions of installations worldwide, it is no wonder phpBB™ is one of the most popular ways to create and support a community." - phpBB.com

"MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia. It is now used by several other projects of the non-profit Wikimedia Foundation and by many other wikis, including this website, the home of MediaWiki." - MediaWiki.org

"With Gallery you can easily create and maintain albums of photos via an intuitive interface. Photo management includes automatic thumbnail creation, image resizing, rotation, ordering, captioning, searching and more. Albums and photos can have view, edit, delete and other permissions per individual authenticated user for an additional level of privacy.

It's great for communities - give accounts to your friends and family and let them upload and manage their own photos on your website!" - gallery.menalto.com

"Drupal is a free software package that allows an individual or a community of users to easily publish, manage and organize a wide variety of content on a website. Tens of thousands of people and organizations are using Drupal to power scores of different web sites..." - Drupal.org

"Joomla is an award-winning content management system (CMS), which enables you to build Web sites and powerful online applications. Many aspects, including its ease-of-use and extensibility, have made Joomla the most popular Web site software available. Best of all, Joomla is an open source solution that is freely available to everyone." - Joomla.org

"Zen Cart™ truly is the art of e-commerce; free, user-friendly, open source shopping cart software. The ecommerce web site design program is being developed by group of like-minded shop owners, programmers, designers, and consultants that think ecommerce web design could be and should be done differently." - Zen-cart.com