Administration

• Webmin – Web based interface for system administration of Unix/Linux
• Virtualmin – Webmin module for web hosting
• Usermin – Webmin module for webmail and user control panel

CMS, Forum, Blog, Wiki

• WordPress – Blog and publishing system

Dev

• MantisBT – Mantis Bug Tracker is a free and open source web-based bug tracking system
• Git – Git is an open source version control system