UptimeRobot - Free Website Uptime Monitoring
Zenoss Core - Zenoss (Zenoss Core) is an open source application, server and network management platform based on...
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
LANState - LANState - Network Mapper and Device Monitor
Uptime Kuma - A fancy self-hosted monitoring tool.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services