NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Server Monitoring - A server monitor is a tool that monitors your server and website on close intervals and reports to you if it's inaccessible.
Pingbreak - Pingbreak is an original and free monitoring service.
mackerel.io - mackerel.io is a server and application performance monitoring platform.
DownDetector - A simple service to tell you whether a website is down.