RackTables - Racktables is a nifty and robust solution for datacenter and server room asset management.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
DCImanager - DCImanager is a platform for managing physical equipment. Connect any physical equipment to a single platform. Use the platform to manage your servers, switches, PDU as well as physical and virtual networks.
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.
NetBox - NetBox is an open source web application designed to help manage and document computer networks. NetBox was developed specifically to address the needs of network and infrastructure engineers.
Prometheus - An open-source systems monitoring and alerting toolkit.