HHVM - HHVM is an open-source virtual machine designed for executing programs written in Hack and PHP.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
CloudPhysics - Analytics for virtual datacenters
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Skylight.io - Skylight is a smart profiler data detective code whisperer bug catcher flag raiser for rails apps.
Linux Dash - A drop-in, low-overhead monitoring web dashboard for a Linux machine.