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.
DownDetector - A simple service to tell you whether a website is down.
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.
Pingbreak - Pingbreak is an original and free monitoring service.
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.
UptimeRobot - Free Website Uptime Monitoring