Zabbix - Track, record, alert and visualize performance and availability of IT resources
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases
Statista - The Statistics Portal for Market Data, Market Research and Market Studies
Netdata - Monitor your servers, containers, and applications, in high-resolution and in real-time.
Montecarlito - MonteCarlito is a free Excel-add-in to do Monte-Carlo-simulations.
eZ Server Monitor - eZ Server Monitor is a script for monitoring Linux servers.