What are some alternatives?
When comparing ZPanel and Plesk, 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.
Google App Engine - A powerful platform to build web and mobile apps that scale automatically.
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.
Webmin - Webmin, Usermin, Virtualmin, Cloudmin, Linux, System Administration
Dokku - Docker powered mini-Heroku in around 100 lines of Bash
Virtualmin - It includes numerous features not in the GPL version (like script installers, resellers, HTML editor, an improved UI, mobile access, spam and virus filtering).