WordPress
"WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. More simply, WordPress is what you use when you want to work with your blogging software, not fight it. WordPress is both free and priceless at the same time."

Official Website: http://www.wordpress.org
Demo: http://demo.opensourcecms.com/wordpress/
Admin Demo: http://demo.opensourcecms.com/wordpress/wp-login.php
Username: admin
Password: demo


Geeklog
"Geeklog is a PHP/MySQL based application for managing dynamic web content. "Out of the box", it is a blog engine, or a CMS with support for comments, trackbacks, multiple syndication formats, spam protection, and all the other vital features of such a system."

Official Website: http://www.geeklog.net
Demo: http://demo.geeklog.net/index.php
Admin Demo: http://demo.geeklog.net/admin/moderation.php
Username: Admin
Password: password


Mambo Open Source
A professional level yet easy to use Content Management System featuring inline WYSIWYG content editors, newsfeeds, syndicated news, banners, mailing users, links manager, statistics, content archiving, date based content, 20 languages, modules and components.

Official Website: http://www.mambo-foundation.org


phpBB
A widely-popular open-source bulletin-board package, works well, simple user interface and admin panel, clean look, scales well, and can be customized.

Official Website: http://www.phpbb.com
Demo: http://www.phpbb.com/community/


SMF
Elegant. Effective. Powerful. Free. 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.

Official Website: http://www.simplemachines.org
Demo: http://www.simplemachines.org/community/


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.

Official Website: http://www.coppermine-gallery.net
Demo: http://coppermine-gallery.net/demo/cpg14x/


Gallery
An Image Gallery system featuring albums within albums, thumbnailing specific picture area, captions, rotate, reorder pictures, album-based attributes, album mirroring.

Official Website: http://gallery.menalto.com
Demo: http://gallery.menalto.com/gallery/demosites/