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.
statuspage - A simple self-hosted status page site with an API written in Django under the BSD license.
UptimeRobot - Free Website Uptime Monitoring
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
YUNO - YUNO is a web data driven company focussed on solving customer challenges in web data discovery and extraction space.