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.
Prometheus - An open-source systems monitoring and alerting toolkit.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
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.
StatsD - Simple daemon for easy stats aggregation.
Hosted Graphite - Graphite as a service
SolarWinds Server & Application Monitor - SolarWinds Server & Application Monitor provides deep visibility into health, availability and performance of enterprise applications and multi-vendor servers.