ManageEngine Exchange Reporter Plus is a reporting, auditing, monitoring, and content search tool for Exchange Server, Exchange Online, and Skype for Business. It offers over 450 comprehensive and intuitive reports on all aspects of Exchange, like mailboxes, Outlooks Web Access (OWA), ActiveSync, email traffic, storage, distribution lists, public folders, and more. It audits every user activity, and stores the logs for an indefinite period. This solution also helps you monitor Exchange Server services and endpoints 24/7, and sends real-time alert notifications via email or SMS.
UptimeRobot - Free Website Uptime Monitoring
Prometheus - An open-source systems monitoring and alerting toolkit.
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.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
StatsD - Simple daemon for easy stats aggregation.