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.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Oh Dear! - Website monitoring, without the hassle
Freshworks - Delight your customers, from wherever you are
Troops Slack Sales Gong - The most customizable Salesforce Gong in Slack
updown.io - updown.io is a HTTP(S)/TCP/ICMP monitoring service which checks periodically the URL you want and reports back any anomaly.