Zabbix is an enterprise-class open source distributed monitoring solution.

Zabbix is software that monitors numerous parameters of a network and the
health and integrity of servers. Zabbix uses a flexible notification
mechanism that allows users to configure e-mail based alerts for virtually
any event. This allows a fast reaction to server problems. Zabbix offers
excellent reporting and data visualisation features based on the stored
data. This makes Zabbix ideal for capacity planning.

WWW: https://www.zabbix.com/