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.

UptimeRobot - Free Website Uptime Monitoring

StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring

Monitis - Monitis specializes in IT monitoring, covering server, website, app monitoring and more within an all-in-one monitoring platform.

StatusPage.io - StatusPage.io is the best way for web infrastructure, developer API, and SaaS companies to get set up with their very own status page in minutes. Integrate public metrics and allow your customers to subscribe to be updated automatically.

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.