Suprvise is a real-time health monitoring and errors tracking dashboard.
Avospy - Avospy offers website monitoring, instant alerts, SEO analyzer, spying competition to get precise statistics about error on the website.
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
UptimeRobot - Free Website Uptime Monitoring
Oh Dear! - Website monitoring, without the hassle
Monika - Get notified from whatever app you want (Telegram, Slack, etc) when your website or API is down based on HTTP status code or content of the response, or when your server is slow.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting