Zabbix - Track, record, alert and visualize performance and availability of IT resources

OnlineOrNot - Uptime monitoring for software teams (even teams of one).

FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.

Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.

Abusix - Abusix is a network security company that offers email threat protection and network abuse handling solutions to ISPs, Telcos, and Cloud/Hosting Providers to stop the rapidly growing problem of email-borne threats and other kinds of network abuse.

Uptime Kuma - A fancy self-hosted monitoring tool.