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.
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StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UXPin + Slack - Keep tabs on design projects in Slack
Better Uptime - We call you when your website goes down