Question: what tools have you used for infrastructure monitoring?
Answer: SolarWinds Network Performance Monitor (FREE TRIAL) ... 
Datadog Infrastructure Monitoring (FREE TRIAL) ... 
SolarWinds Server & Application Monitor (FREE TRIAL) ... 
Paessler PRTG Network Monitor. ... 
Site24x7 Server Monitoring. ... 
Atera. ... 
ManageEngine OpManager. ... 
Nagios X.