The best open source alternative to cPanel is Vesta Control Panel, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to cPanel and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to cPanel are Webmin (Free, Open Source), ZPanel (Free, Open Source), Cloudron (Freemium, Open Source) and CentOS Web Panel (Freemium, Open Source).
Hestia Control Panel is an open source web server control panel with an easy-to-use interface as a lightweight alternative to cPanel, Plesk, etc. Hestia It's also the continuation of the development of Vesta Control Panel.
aaPanel provides the one-click function such as one-click install LNMP/LAMP developing environment and software. Our main goal is helping users to save the time of deploying, thus users just focus on their own project that is fine.
ApisCP is a hosting platform for modern websites. Multi-tenant with support for PHP, Ruby, Node, Python, and Go projects. Automatic Let's Encrypt, passive monitoring, threat-deterrence, and Ansible-driven platform checks make administration easy.
SolidCP is a complete hosting Control Panel for hosting and IT providers to manage many enterprise software such as: Server 2014, MySQL, MariaDB, Microsoft Exchange, Sharepoint, Lync/ Skype for Business , Webdav, RDS Services, and Hyper-V.