UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
StatusOk (Open source) - Get notified through Slack/email when your website is down
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Troops Slack Sales Gong - The most customizable Salesforce Gong in Slack
Robodash - Cron and uptime monitoring dashboard
UXPin + Slack - Keep tabs on design projects in Slack