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.
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.
UptimeRobot - Free Website Uptime Monitoring
Unitas Atlas - Unitas Atlas is a system monitoring platform that allows you to monitor and view all the parameters of your IT systems, internet, cloud storage, and many others.
SolarWinds Pingdom - With website monitoring from Pingdom, you will be the first to know when your website is down. No installation required. 14-day free trial.
Service Uptime - Free website monitoring service checks your site from worldwide locations and alerts you instantly via email or SMS when it becomes unavailable. Detailed uptime statistics and performance reports available.