perf - Perf is a simple app monitoring solution paired with meaningful alerts.

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.

Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.

Dynatrace - Cloud-based quality testing, performance monitoring and analytics for mobile apps and websites. Get started with Keynote today!

Loado - Loado is an easy-to-use monitoring tool for websites that helps developers, products, and marketers to improve user conversions and optimize SEO strategies.

Catchpoint - Catch issues before they impact your business