Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
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.
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.
UptimeRobot - Free Website Uptime Monitoring
Troops Slack Sales Gong - The most customizable Salesforce Gong in Slack
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.