Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
Prometheus - An open-source systems monitoring and alerting toolkit.
UptimeRobot - Free Website Uptime Monitoring
Fluent Bit - Data collector and log forwarder.
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue