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.
UptimeRobot - Free Website Uptime Monitoring
cState - 🔥 Free & open source under MIT!
Indatus - Indatus – A Creative Editor Making Your Photos Gorgeous an all-in-one photo-editing application developed by Thang Dinh.
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.
Pulsetic - Pulsetic makes monitoring your SaaS products simple and easy with Status badges to show the status of the site without changing pages and you will also be alerted when your site goes down.