Coverity Scan - Find and fix defects in your Java, C/C++ or C# open source project for free
Zabbix - Track, record, alert and visualize performance and availability of IT resources
SonarQube - SonarQube, a core component of the Sonar solution, is an open source, self-managed tool that systematically helps developers and organizations deliver Clean Code.
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.
ReSharper - ReSharper is a productivity tool for visual studio that provides tools and features to help you manage your code.
Prometheus - An open-source systems monitoring and alerting toolkit.