Troops Slack Sales Gong - The most customizable Salesforce Gong in Slack
UptimeRobot - Free Website Uptime Monitoring
Attentive - A marketing channel, built for retail & e‑commerce
Uptime Kuma - A fancy self-hosted monitoring tool.
StatusOk (Open source) - Get notified through Slack/email when your website is down
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.