Suprvise is a real-time health monitoring and errors tracking dashboard.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
Monitority - Simple website monitoring service (sms/email alerts)