What are some alternatives?
When comparing ISPConfig and Virtualmin, you can also consider the following products
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.
Webmin - Webmin is a web-based interface for system administration for Unix.
CentOS Web Panel - Free CentOS Linux Web Hosting control panel designed for quick and easy management of (Dedicated & VPS) servers without of need to use ssh console for every little thing
Vesta Control Panel - – What I love about Vesta is that it's fast and easy to use
Ajenti - Web administration panel for servers and custom hardware
Sentora - Sentora is an open-source web hosting control panel built specifically to work on a variety of Linux distributions. Sentora is licensed under the GPL and is a separately maintained fork of the original ZPanel project.