UptimeRobot - Free Website Uptime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Uptime Kuma - A fancy self-hosted monitoring tool.
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.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Troops Slack Sales Gong - The most customizable Salesforce Gong in Slack