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.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
UptimeRobot - Free Website Uptime Monitoring
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.
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.
Troops Slack Sales Gong - The most customizable Salesforce Gong in Slack