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.
Is It Down Right Now? - Network & Admin
Oh Dear! - Website monitoring, without the hassle
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
Freshworks - Delight your customers, from wherever you are
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.