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