Chaos Monkey is an open source software tool for DevOps created by Adrian Cockroft at Netflix that tests computer systems. The software works by intentionally creating different failures and tests the ability to survive them.
The purpose of Chaos Monkey is to test the cloud to find failures and have the opportunity to fix them before they happen on their own. Because its purpose is to alert someone of failure or abnormalities in the system, the service is usually only run during business hours or if there will be an engineer around to troubleshoot and fix issues that arise.
Source: Wikipedia