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
Zabbix - Track, record, alert and visualize performance and availability of IT resources
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Fluent Bit - Data collector and log forwarder.