Moodle:
A Course Management System designed to help educators create quality online courses. Available in currently 34 languages, featuring WYSIWYG HTML Editor, teacher has full control over all settings for a course, flexible array of course activities (Forums, Journals, Quizzes, Resources, Choices, Surveys, Assignments, Chats, Workshops), user logging and tracking, mail integration and much more.
http://moodle.org/

Noahs Classifieds:
A classifieds system featuring categories and subcategories in unlimited depth, image upload for categories and classifieds, variable fields per categories, customizable email notifications, locking categories, classifieds approval, auto-thumbnail generation, classifieds management by user, send classified to a friend.
http://classifieds.phpoutsourcing.com/

Open-Realty:
Real Estate listing system featuring attachments, flexible search, template system, Yahoo Maps interface.
http://www.open-realty.org/

phpAdsNew::
A highly professional complete ad server featuring different types of ads, display based on keywords, zones, hour of the day, the day of the week, extensive statistics, client statistics.
http://phpadsnew.com/

PHPauction:
The GPL version of PHPauction featuring email notification of bids, attachments, reserve price, minimum bids, standard and dutch auctions, bid history, send auction to a friend, highest bids, auctions ending soon.
http://www.phpauction.org/

phpFormGenerator:
A form generator featuring up to 100 form fields, all kind of input fields incl. file upload, customizable fields attributes, send submitted data to an email address or store them in a database, admin panel.
http://phpformgen.sourceforge.net/

phpLinks:
A Link Directory featuring categories, reviews, popular links, new links and links approval.
http://sourceforge.net/projects/phplinks/

WebCalendar:
A very powerful webcalendar featuring private and public calendars.
http://webcalendar.sourceforge.net/

Just Added!

Coppermine Photo Gallery:
An Image Gallery system featuring categories and albums, thumbnails and intermediate size pics, search feature, new and random pictures, user management (private galleries, groups), user comments, e-cards feature, slideshow viewer.
http://coppermine.sourceforge.net/

FAQ MasterFlex:
Unlimited categories/questions/answers, web-based administration.
http://www.technetguru.com/faqmasterflex.php

phpCOIN:
A full billing/invoicing application, perfect for webhosting resellers.
http://www.phpcoin.com/

Advanced Poll:
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. Supports textfile and mySQL database. http://www.proxy2.de/scripts.php/

Mambo Open Source:Content management system.
Mambo Open Source is the engine behind your website that provides the ability to simplify the creation of content. http://www.mamboserver.com/

PHPSurveyor:
The PHP Surveyor is a set of PHP scripts that interact with MySQL to develop surveys, publish surveys and collect responses to surveys.
http://phpsurveyor.sourceforge.net/

PhpWiki:
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.
http://phpwiki.sourceforge.net/

TikiWiki:
Tiki CMS/Groupware (aka TikiWiki) is a powerful open-source Content Management System (CMS) and Groupware that can be used to create all sorts of Web applications, Sites, Portals, Intranets and Extranets. TikiWiki also works great as a Web-based collaboration tool.
http://tikiwiki.org/tiki-view_articles.php

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

phpWCMS:
The phpwcms is an Open Source web content management system released under the GNU General Public License. You can use it for free. phpwcms is optimized for fast and easy setup and works on any standard webserver platform that supports PHP/MySQL and was tested successfully on Windows 2000/XP, MacOSX and LINUX. phpwcms is perfect for professional, public and private users. It is very easy to learn and gives you the flexibility to separate layout and content. Lots of powerful but simple implemented features