Is It Down Right Now? - Network & Admin
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
Pingbreak - Pingbreak is an original and free monitoring service.