Mature, Production Ready Open-Source Software

This will be an ever-growing list of mature and production-ready open-source software:

• Linux, Kernel | http://www.kernel.org/
• Debian, OS | http://www.debian.org/
• Apache, web-server | http://httpd.apache.org/
• PostgreSQL, DB | http://www.postgresql.org/
• MySQL, DB | http://www.mysql.com/
• Drupal, CMS | http://drupal.org/
• WordPress, CMS | http://wordpress.org/
• Joomla, CMS | http://www.joomla.org/
• Xen, VM hypervisor | http://xen.org/
• PHP,  Scripting language | http://www.php.net/
• Red Hat, OS, | http://www.redhat.com/
• Cent OS, OS | http://www.centos.org/ (Red Hat minus the branding & artwork)
• MediaWiki, wiki server | http://www.mediawiki.org/