b2evolution is probably the most comprehensive blog engine you can find! It includes almost any feature you could expect from a blog tool, and more !
Homepage: http://b2evolution.net/

Mambo is a full-featured content management system that can be used for everything from simple websites to complex corporate applications.
Homepage: http://mamboserver.com

PHP Nuke - open source professional portal system
Homepage: http://www.phpnuke.org/

A WikiWikiWeb is a web site where anyone can edit the pages through an HTML form. Linking is done automatically on the server side; all pages are stored in a database.
Homepage: http://phpwiki.sourceforge.net/

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 customisable, and in constant development.
Homepage: http://www.e107.org/

vtiger CRM is an Open Source CRM software mainly for small and medium businesses. vtiger CRM is built over proven, fast, and reliable LAMP/WAMP (Linux/Windows, Apache, MySQL, and PHP) technologies and other open source projects.
Homepage: http://www.vtiger.com/

>>Forums

XMB is a fully scalable, highly customizable Open Source message board software. XMB has a user-friendly interface, simple set up, stable performance, and tons of features. Based on the powerful PHP language and the MySQL database, XMB is a very powerful combination, suitable to everyone's needs.
Homepage: http://www.xmbforum.com

SMF is all of the above. SMF is a next-generation community software package and is jam-packed with features, while at the same time having a minimal impact on resources.
Homepage: http://www.simplemachines.org/

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 your choice of MySQL, MS-SQL, PostgreSQL or Access/ODBC database servers, phpBB is the ideal free community solution for all web sites.
Homepage: http://www.phpbb.com/

>>E-Commerce

A free software package originally designed for web-hosting resellers to handle clients, orders, invoices, notes and helpdesk, but no longer limited to hosting resellers.
Homepage: http://www.phpcoin.com

CubeCart is an eCommerce script written with PHP & MySQL. With CubeCart you can setup a powerful online store as long as you have hosting supporting PHP and one MySQL database.
Homepage: http://www.cubecart.com/site/home/

SugarCRM is a suite of business automation tools for managing your marketing, sales and customer service operations. From leads to contacts, opportunities to cases, the Sugar Suite helps you track and gain insight into your customers.
Homepage: http://www.sugarcrm.com/

phpAdsNew is an open-source ad server, with an integrated banner management interface and tracking system for gathering statistics. With phpAdsNew you can easily rotate paid banners and your own in-house advertisements. You can even integrate banners from third party advertising companies.
Homepage: http://phpadsnew.com/two

osCommerce is an online shop e-commerce solution under on going development by the open source community. Its feature packed out-of-the-box installation allows store owners to setup, run, and maintain their online stores with minimum effort and with absolutely no costs or license fees involved.
Homepage: http://www.oscommerce.com/

>>Support Systems

Help Center Live is a `Live` help desk system written in PHP using a MySql database backend that features Live Support, Trouble Tickets and FAQ within one project.
Homepage: http://www.helpcenterlive.com/

phpMyChat is an easy-to-use multi-room chat based on PHP and a database.
Homepage: http://phpmychat.sourceforge.net

phpMyFAQ 1.5.0 is a multilingual, completely database-driven FAQ-system. For the time being a MySQL database (support for other databases is under development) is used to store all data, PHP 4.1.0 (or higher) is needed in order to access this data. phpMyFAQ also offers a Content Management-System, flexible multi-user support, a news-system, user-tracking, language modules, templates, extensive XML-support, PDF-support, a backup-system and an easy to use installation script.
Homepage: http://www.phpmyfaq.de/

A multi-user, multi-operator, multi-department live Help support chat system that allows the operators of the websites to monitor their visitors as they are browsing the site and proactively open a chat session with the visitor.
Homepage: http://www.craftysyntax.com/livehelp/

>>Galleries

Coppermine Photo Gallery is a picture gallery script. Users can upload pictures with a web browser (thumbnails are created on the fly), rate pictures, add comments and send e-cards. The admins can manage the galleries and batch add pictures that have been uploaded on the server by FTP. Support for multimedia files has been added recently.
Homepage: http://coppermine.sourceforge.net/

Gallery is a web based software product that lets you manage your photos on your own website. You must have your own website with PHP support in order to install and use it. 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 can have read, write and caption permissions per individual authenticated user for an additional level of privacy.
Homepage: http://gallery.menalto.com/

>>Misc. Scripts

Advanced Poll is a polling system with powerful administration tool. It features: multiple polls, templates, unlimited options, multi-language support, IP-Logging, IP-Locking, cookie support, comment feature, vote expire feature, random poll support and more.
Homepage: http://proxy2.de/scripts.php

This php Site Recommendation script will allow your visitors to enter their contact information and recommend your site to as many recipients as you specify (only one required). The default messages are easily modified. Look and feel is customizable via the stylesheet. You can choose to be notified when your site has been recommended...notification will show recipient(s) AND the message. Email addresses are hidden so they will not be harvested by spammers. Also contains error checking and validation and does not require database access.
Homepage: http://www.gatequest.net/products/Site_Recommender/

Homepage: http://www.clixnetwork.com/products/php-my-counter/

Access and manage your private, shared or public bookmarks from any computer and browser connected to the Internet! Create personal, family, team, company or enterprise-wide bookmarks with flexible access rules, permissions and customizable design!
Homepage: http://www.sitebar.org/

 Homepage: http://webcards.sourceforge.net

PHP scripts to let non-technical users create surveys, administer surveys, gather results, view statistics. All managed online after database initialization. MySQL database backend.
Homepage: http://sourceforge.net/projects/phpesp/

PHProjekt is a modular application for the coordination of group activities and to share informations and document via intranet and internet.
Homepage: http://www.phprojekt.com/

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.
Homepage: http://www.k5n.us/webcalendar.php/