Network devices, as in appliances like a Cisco firewall, in contrast to Windows/Linux hosts, usually support SNMP out of the box and that is normally suitable to monitor them adequately (perhaps in combination with SNMP traps). Also, you couldn't install an agent in most cases. There are SNMP plugins for Nagios.
Windows/Linux hosts, as pointed in some other comment, require something to provide monitoring. In Windows this can be WMI, which is built in (although it has its quirks- IIRC Microsoft's own monitoring solution didn't use WMI and used an NCPA-like agent when I looked at that). On Linux you can use ssh to invoke plugins without an agent, or other solutions like snmpd, but they also are not perfect. I'd recommend using a monitoring agent on Windows and
omegleshagle Linux.