Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
SpeedFan - Hardware monitor for Windows that can access digital temperature sensors located on several 2-wire SMBus Serial Bus. Can access voltages and fan speeds and control fan speeds. Includes technical articles and docs.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
xScan - xScan is an application for viewing the behavior of your computer and Mac.
Prometheus - An open-source systems monitoring and alerting toolkit.