Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
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.
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
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.