RedGate SQL Monitor - SQL Monitor helps you and your team find issues – before they become problems
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.
Redgate SQL Data Compare - Use Redgate SQL Data Compare to compare and synchronize static, lookup, and reference data in your SQL Server database - try it free
Prometheus - An open-source systems monitoring and alerting toolkit.
Open DBDiff - A database comparison tool for Microsoft SQL Server 2005+ that reports schema differences and...
Nagios - Complete monitoring and alerting for servers, switches, applications, and services