OnlineOrNot - Uptime monitoring for software teams (even teams of one).

UptimeRobot - Free Website Uptime Monitoring

Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.

StatusPage.io - StatusPage.io is the best way for web infrastructure, developer API, and SaaS companies to get set up with their very own status page in minutes. Integrate public metrics and allow your customers to subscribe to be updated automatically.

Uptime Kuma - A fancy self-hosted monitoring tool.

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.