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.
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.
Rigor Monitoring & Optimization - Rigor Monitoring & Optimization is a website monitoring and performance measuring software solution that allows you to view the website from a customer perspective and conduct various tests to enhance the performance and working of the website.
Pingbreak - Pingbreak is an original and free monitoring service.
ScienceLogic - ScienceLogic simplifies data center, cloud, and network monitoring with its all-in-one platform.
UptimeRobot - Free Website Uptime Monitoring