Is there currently anyway to monitor CPU usage with Nagios3 that doesn't involve SNMP setup? I want to setup notifications if I'm going above a certain threshold, such as 55% of the CPU time is being used up by the system.
The only other way other than SNMP to get remote information off a system that isn't checking a network based service (ie- HTTP, SMTP, POP, etc) is to use NRPE which means installing the NRPE daemon on the client machine and configuring the checks you want to be able to run locally over NRPE. NRPE uses SSL to communicate between machines over a port, 5666 by default, and can run any Nagios plugin locally on the client and transmit the result back to Nagios as if it had been run on the Nagios server.
I have Nagios configured to check available disk space and free memory using HTTP monitoring and regexes instead of SNMP. I have a PHP script on the servers that prints the free disk space or free memory, and I use Nagios' HTTP monitoring to make sure the result matches
[0-8][0-9]\% to make sure I'm under 90% usage.