What are some alternatives?
When comparing Virtualmin and ZPanel, 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.
ISPConfig - ISPConfig 3 is an open source hosting control panel for Linux.
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.
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
Plesk - Plesk puts all the automation, security, and technical tools an IT professional needs in one simple and easy to use dashboard.
Webmin - Webmin is a web-based interface for system administration for Unix.