Zabbix - Track, record, alert and visualize performance and availability of IT resources
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
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.
GLPI - GLPI is the Information Resource-Manager with an additional Administration- Interface.
Prometheus - An open-source systems monitoring and alerting toolkit.
Snipe-IT - Managing assets with a Google doc or a shared Excel spreadsheet is more common than you think.