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.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Ubersuggest - Want more traffic? Ubersuggest shows you how to win the game of SEO. Just type in a domain or a keyword to get started.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
BytesMatter - Web monitoring and analytics through the eyes of your users
Better Uptime - We call you when your website goes down