UptimeRobot - Free Website Uptime Monitoring

Suprvise - Know instantly when your sites are down.

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.

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.

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.

Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.