Ixchariot - Instant performance assessment of complex networks from pre- to post-deployment.
UptimeRobot - Free Website Uptime Monitoring
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
Uptime Kuma - A fancy self-hosted monitoring tool.
PsPing - PsPing is a command-line utility for measuring network performance.
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.