CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
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.
Google Project Shield - Google's DDoS protection service
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
SSL Alerts - Free service to send SSL certificate expiration alerts
Better Uptime - We call you when your website goes down