Content Management

Joomla
Joomla! is one of the most powerful Open Source Content Management Systems on the planet. It is used all over the world for everything from simple websites to complex corporate applications. Joomla! is easy to install, simple to manage, and reliable.

Mambo
Mambo is a full-featured content management system that can be used for everything from simple websites to complex corporate applications. Features: a large and healthy user and developer community, content approval for registered users, online help, an advanced admin system, powerful templates, and more.

Drupal
Drupal is software that allows an individual or a community of users to easily publish, manage and organize a great variety of content on a website. Drupal includes features to enable content management systems, blogs, collaborative authoring environments, forums, newsletters, picture galleries, file uploads and download and much more.

e107
e107 is a content management system written in php and using the popular open source mySQL database system for content storage. It's completely free and totally customizable, and in constant development.

Weblogs

WordPress
WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. WordPress features: Instant publishing, comment spam protection, themes, content management, easy upgrades, post-by-email, and more.

b2evolution
b2evolution is probably the most comprehensive blog engine you can find! All the features you can expect are there, including: Advanced browsing (paged, calendar, categories, search...), extended & multipaged posts, image/file upload, multiple and sub-categories, draft and quick publishing, spell checker, RSS feeds etc.

Image Galleries

Coppermine
Coppermine is a multi-purpose fully- featured and integrated web picture gallery script written in PHP using GD or ImageMagick as image library with a MySQL backend.

ZenPhoto
Zenphoto is an answer to lots of calls for an online gallery solution that just makes sense. After years of bloated software that does everything and your dishes, ZenPhoto just shows your photos, simply.

Wikis

MediaWiki
MediaWiki is wiki software that is used by Wikimedia projects and others. It is an implementation of a wiki, a content pool that anyone can freely edit. It's designed to handle a large number of users and pages without imposing too rigid a structure or workflow.

Discussion Boards

php-BB
phpBB is a high powered, fully scalable, and highly customizable Open Source bulletin board package. phpBB has a user-friendly interface, simple and straightforward administration panel, and helpful FAQ. Based on the powerful PHP server language and MySQL database server, phpBB is the ideal free community solution for all websites.

SMF
Elegant, powerful, and easy SMF is a next-generation community forum software package that is jam-packed with features, while at the same time having a minimal impact on resources.

Guestbooks

Gbook
Setup a guestbook for your website with this free PHP script. You can set it up to process smileys, it can send you an email notification when someone signs your guestbook, and you can delete inappropriate messages.

Groupware

WebCalendar
WebCalendar is a PHP-based calendar application that can be configured as a single- user calendar, a multi-user calendar for groups of users, or as an event calendar viewable by visitors.

E-Commerce

ZenCart
ZenCart truly is the art of e-commerce; a free, user-friendly, open source shopping cart system. The software is being developed by group of like-minded shop owners, programmers, designers, and consultants that think e-commerce could be and should be done differently.

osCommerce
osCommerce is an online shop e-commerce solution that offers a wide range of out-of-the-box features that allow online stores to be setup fairly quickly with ease. It is an e-commerce framework solution that not only remains easy to setup and maintain, but also makes it easier for store administrators to present their stores to their customers with their own unique requirements.

RSS

Gregarius
Gregarius is a web-based RSS/RDF/ATOM feed aggregator, designed to run on your web server, allowing you to access your news sources from wherever you want. Features: Completely web-based (runs on your web server), simple, password-protected, administration and configuration, AJAX powered item tagging and more.

Small Scripts

php-FormGenerator
phpFormGenerator is an easy-to-use tool to create reliable and efficient web forms in a snap. No programming of any sort is required. Just follow along the phpFormGenerator wizard and at the end, you will have a fully functional web form!