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.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Prometheus - An open-source systems monitoring and alerting toolkit.
NewRelic - New Relic is a Software Analytics company that makes sense of billions of metrics across millions of apps. We help the people who build modern software understand the stories their data is trying to tell them.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Zabbix - Track, record, alert and visualize performance and availability of IT resources
StatsD - Simple daemon for easy stats aggregation.