Virtualmin - Webmin

Virtualmin is a very powerful and light control panel for Unix-based and of course Linux servers, written mostly in perl, being one of the favorites tools of administrators

The reason is simple, it very expandable and does not affect operating system on which is installed, unlike for example radical changes taking place by cPanel. The management is offered by it's own web server providing a web-interface for setting up all available services (Apache, NGINX, MySQL, PHP, mailserver, etc) and config files of the system.

It is very secure by it's design (only a small amount of security issue in the last 10 years) in addition it's robust architecture offers the ability for easy plugin development and customization. It also remains modern (NGINX webserver support, DKIM signatures on e-mails) as it is frequently updated with new features.

Two big projects based on Webmin are Usermin and Virtualmin.

Usermin, consists of a subset of features (like webmail access) and appeals to users.
While Virtualmin, is a complete web hosting control panel (like cPanel/WHM) which enables to host websites per domain. It appeals to administrators and advanced users/developers managing and developing websites. It supports services management running on the server, automatic backups, locally, on an other server or on Cloud. Offers import tool from cPanel or plesk accounts and of course creating quotas on disk space, bandwitdh etc for users.

In other words (because it is impossible to describe all the features of Webmin - Virtualmin in an article) Virtualmin, is a competitor of commercial control panels, with the great advantage that it is supported by open source community and being available for free.

UPDATE: virtualmin installation guide on debian

Screenshots: 
Webmin configuration screenshot
Webmin usermin configuration screenshot
Virtualmin overview screenshot
Virtualmin plans screenshot
Virtualmin backup scheduler screenshot
Virtualmin cpanel plesk migration screenshot
Virtualmin services configuration screenshot