Here is the list, it's long and it changes with new updates and additions every few days.
Blogs
Serendipity PHP-powered, flexible Blogging/CMS application
Open Blog Kick-ass Blog application built using the CodeIgniter PHP Framework
b2evolution b2evolution is a powerful blog tool
LifeType LifeType is an open-source blogging platform with support for multiple blogs and users in a single installation
WordPress WordPress is a state-of-the-art publishing platform with a focus on aesthetics, web standards, and usability.
Nucleus Nucleus CMS is a flexible and easy to install content management system, aka blog software.
Dotclear Dotclear is an open-source web publishing software
Textpattern Textpattern is a flexible, elegant and easy-to-use content management system.
WordPress MU Ever dream of running hundreds of thousands of blogs with a single install of WordPress ?
eggBlog eggBlog is the free php & mysql blog software package
Pixie The Small, Simple, Site Maker
Portals/CMS
Zikula Zikula is a Web Application Toolkit, which allows you to run impressive websites and build powerful online applications.
Xoops Xoops is an easy to use dynamic web content management system written in PHP
Joomla Joomla is an award-winning CMS, which enables you to build Web sites and powerful online applications
Website Baker Website Baker is a PHP-based Content Management System
sNews sNews is extremely lightweight, simple and customizable
Geeklog Geeklog is a PHP/MySQL based application for managing dynamic web content.
Drupal Drupal is an open source content management platform
Mambo Mambo is a full-featured content management system
PHP-Nuke It is a CMS that integrates in its inside all the instruments that are used to create, in a broad sense, an information portal.
Pligg Pligg is an open source Content Management System
MODx MODx helps you take control of your online content.
Dolphin The worlds most advanced community software
Forums
phpBB The most widely used Open Source forum solution
AEF AEF – Advanced Electron Forum is a free bulletin board software written in PHP and MySQL with loads of features
XMB XMB is a lightweight PHP forum software with all the features you need to support a growing community
UseBB UseBB is a light and Open Source forum software
MyBB Its the easy to use, multilingual, powerful, feature packed, free forum software
Phorum The Original PHP and MySQL Forum Software
PunBB PunBB is a fast and lightweight PHP-powered discussion board.
SMF SMF is a free, professional grade software package that allows you to set up your own online community
FluxBB FluxBB is a free open source forum application designed to be fast, light and user friendly
Image Galleries
Gallery Gallery is an open source web based photo album organizer
LinPHA LinPHA is an easy to use, multilingual, flexible photo/image archive/album/gallery
Coppermine Coppermine is a multi-purpose fully-featured and integrated web picture gallery script written in PHP.
TinyWebGallery TinyWebGallery is a free php based photo album / gallery
Piwigo Piwigo is a photo gallery software for the web
Pixelpost Pixelpost is an open-source, standards-compliant, multi-lingual, fully extensible photoblog application for the web
ZenPhoto a simpler web photo gallery
phpAlbum Create your personal Photo Album / Gallery in just seconds
Shutter Shutter allows you to store and share your photos
4images 4images is as a web based PHP and MySQL for displaying images on the Internet
Wikis
DokuWiki DokuWiki is a standards compliant, simple to use Wiki, mainly aimed at creating documentation of any kind.
MediaWiki MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia
PhpWiki PhpWiki: a PHP WikiWikiWeb
PmWiki PmWiki is a wiki-based system for collaborative creation and maintenance of websites
TikiWiki TikiWiki (Tiki) is your Groupware/CMS (Content Management System) solution
Ad Management
Noahs Classifieds The leading classifieds software on the web!
OpenX The technology platform to power your ad serving needs.
Kamads Kamads Classifieds Comes with easy setup and even easier to use features.
PHPads PHPads is a simple php script that allows management of a banner ad rotations
Calendars
WebCalendar WebCalendar is a PHP-based calendar application
phpicalendar phpicalendar is an open source php application to parse and display shared icalendar-compatible calendars on a website
phpScheduleIt phpScheduleIt is an Open Source web-based reservation and scheduling system
Gaming
BlackNova Traders BlackNova Traders is a web-based, multi-player space exploration game inspired by the popular BBS game of TradeWars
Shadows Rising Shadows Rising is a browser based RPG
Word Search Puzzle Word Search Puzzle is a fun and simple game
Multiplayer Checkers Play checkers and chat with this AJAX-enabled script
Mails
phpList phplist is an open-source newsletter manager
Webinsta Maillist WEBinsta mailing list manager is built to provide a centralized mailing list
poMMo poMMo is versatile mass mailing software
SquirrelMail SquirrelMail is a standards-based webmail package written in PHP
ccMail CcMail is a free, open, powerful Mailing List Manager
Polls and Surveys
LimeSurvey The open source survey application
phpESP Create surveys, administer surveys, gather results, view statistics
CJ Dynamic Poll It is a poll that changes dynamically
EasyPoll Easy Poll is a PHP script to create your own poll
LittlePoll The Amazing Little Poll is a very simple php polling script
Simple PHP Poll Small and lightweight
Aardvark Topsites Aardvark Topsites PHP is a free topsites script built on PHP and MySQL
Advanced Poll Advanced Poll is a polling system with powerful administration tool
Project Management
dotProject dotProject is a volunteer supported Project Management application
phpCollab phpCollab is an open source internet-enabled system for use in projects that require collaboration over the internet.
PHProjekt PHProjekt is a modular application for the coordination of group activities and to share informations and document via the web
NetOffice Online project management with team collaboration, user management, etc
ProjectPier ProjectPier is a Free, Open-Source, self-hosted PHP application for managing tasks, projects and teams through an intuitive web interface
Mantis Bug Tracker MantisBT is a free popular web-based bugtracking system
SugarCRM Sugar CRM is the worlds leading provider of commercial open source customer relationship management (CRM) software.
E-Commerce
Zen Cart Putting the dream of online business ownership within reach of anyone!
Magento Ecommerce platform for growth
Freeway Freeway is by far the largest and most fully featured Open Source eCommerce platform
osCommerce osCommerce is an online shop e-commerce solution
OpenCart The Super Cool Ecommerce Solution
phpShop A Simple Shopping Cart
PrestaShop PrestaShop is professional e-Commerce shopping cart software
CubeCart CubeCart is a fully featured ecommerce shopping cart solution used by over a million store owners around the world
WHMCS WHMCS is an all-in-one client management, billing & support solution for online businesses.
Guest Books
phpBook Allows you to add free guestbook to your site
BellaBook BellaBook is a small and simple PHP guestbook
VX Guestbook Allows you to add free guestbook to your site
Lazarus Lazarus is a free guestbook script written in PHP
RicarGBooK RicarGBooK is a guestbook system written in PHP
Advanced Guestbook Advanced Guestbook is a PHP-based guestbook script
Customer Support
Crafty Syntax Crafty Syntax Live Help (CSLH) is an open source live support solution
Help Center Live Help Center Live is a Live Support tool written in PHP
HESK Hesk is Free PHP Help Desk Software that runs with a MySQL database
osTicket osTicket is a widely-used open source support ticket system
ExoPHPDesk ExoPHPDesk is a free php helpdesk software
phpSupport phpSupport is your customer friendly trouble ticket system
Open Web Messenger Web Messenger is an open-source live support application written in PHP and MySQL
phpOnline phpOnline is a live customer support system
phpMyFAQ phpMyFAQ is an open source FAQ system using PHP
iQDesk iQDesk is a unique software specifically designed for freelancers, small and any type of one-person-business
Music
kPlaylist kPlaylist is a free PHP system that makes your music collection available via the Internet
Podcast Generator Podcast Generator is a free web based podcast publishing script
Impleo Impleo is a simple PHP-script for managing your record collection
Video
Others
FAQMasterFlex This is a free, database-driven web-based application for creating and maintaining Frequently Asked Questions (FAQs) on your web site.
phpLD The most widely used directory script on the internet
Open-Realty Open-Realty is an open source web based real estate listing management application
Moodle Moodle is a Course Management System
WeBid WeBid is an open-source auction script
Claroline Claroline is an Open Source eLearning and eWorking platform
PHPfileNavigator PHPfileNavigator is state-of-the-art, open source web based application to complete manage your files and folders
CodeIgniter CodeIgniter is a powerful PHP framework with a very small footprint
Elgg Elgg empowers individuals, groups and institutions to create their own fully-featured social environment
Libraries
script.aculo.us script.aculo.us provides you with easy-to-use, cross-browser user interface JavaScript libraries to make your web sites and web applications fly
jQuery jQuery is a new kind of JavaScript Library
Ext JS Ext JS is a JavaScript platform for rapid development of cross-browser web apps
Scripty2 scripty2 is a powerful, flexible JavaScript framework
MochiKit A lightweight Javascript library
MooTools MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer
YUI YUI 3 is Yahoos next-generation JavaScript and CSS library
Prototype Prototype is a JavaScript Framework that aims to ease development of dynamic web applications
Rico Javascript for rich internet applications
Dojo Ajax, events, packaging, CSS-based querying, animations, JSON, language utilities, and a lot more
AJS The ultra lightweight JavaScript library
Raphael Raphael is a small JavaScript library that should simplify your work with vector graphics on the web
Sizzle A pure-JavaScript CSS selector engine designed to be easily dropped in to a host library
