UptimeRobot - Free Website Uptime Monitoring
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
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.
SolarWinds System Management - Performance monitoring and management for applications, Websites, servers, storage arrays and virtual infrastructure.
Uptime Kuma - A fancy self-hosted monitoring tool.
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.