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
Uptrends - Uptrends is a website and mobile app that allows you to monitor your website. You can use it to both monitor the security and performance of your website as well as the activity on your site. Read more about Uptrends.
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue
Uptime.com - Everything you require for unified availability monitoring Simple & intuitive industry leading Enterprise-grade features delivered at a fair price, that are continuously improving.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring