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.
Spike.sh - Spike is an incident management platform that alerts via slack, phone and email and automatic escalations to help teams fix infrastructure problems.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Clerk - Clerk.io, the artificial intelligence for e-commerce that knows your customers interests.
Better Uptime - We call you when your website goes down
Sendblue - Programmatically send iMessage and SMS