Sensu is a free and open source monitoring that handles cloud environments. Sensu allows you to monitor servers, services, application health, and business KPIs. Collect and analyze custom metrics and get notified about failures before your users do.

View Sensu On The DevOps Diagram »

Also check out:

The Ultimate DevOps Tool Chest
The DevOps Diagram Generator
DevOps Hot or Not
The Periodic Table of DevOps Tools