NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Cachet - Cachet is a free, open source status page for your API, service or company. Built with all of the features that you'd expect from a status page.
Server Monitoring - A server monitor is a tool that monitors your server and website on close intervals and reports to you if it's inaccessible.
cState - 🔥 Free & open source under MIT!
mackerel.io - mackerel.io is a server and application performance monitoring platform.
Status.io - Status.io is a platform for system status pages and status notifications. Companies rely on Status.io to track outages, schedule planned maintenances, broadcast status notifications, and much more; and their customers love the transparency.