Nagios
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Nagios Core /ˈnɑːɡiːoʊs/, formerly known as Nagios, is a free and open-source computer-software application that monitors systems, networks and infrastructure. Nagios offers monitoring and alerting services for servers, switches, applications and services. It alerts users when things go wrong and alerts them a second time when the problem has been resolved.
since you can install that piece of software on your server without trading anything for that, it can be considered as trade-free.