Webmin - Webmin is a web-based interface for system administration for Unix.
NQE - CIFS/SMB based solutions for embedded / mobile markets.
ClearOS - A very good alternative for Windows Server, for free
Samba - Samba serves as an open source alternative to the traditional domain controller offered by Microsoft.
cPanel - With its first-class support and rich feature set, cPanel & WHM has been the web hosting industry's most reliable, intuitive control panel since 1997.
NFS Manager - NFS Manager is a graphical user interface to control all built-in NFS features of Mac OS X.