Nagios turns out to be both easy and difficult.
There are RPMs in the rpmforge repo. You just need nagios, nagios-plugins and nagios-plugins-nrpe, the latter being required in order to communicate relatively securely with the nrpe daemon that you'll need to run on the remote machines you want to monitor.
On those remote machines you need to install the plugins and nrpe. HOWEVER, you can't really use the RPMs for those because they are set to require nagios itself as a dependancy. Therefore grabbing the source and compiling them is the only option (not that it is difficult!).
In fact it is probably more sensible to compile everything from source. I was just being lazy.
Some useful links:http://wiki.centos.org/HowTos/Nagioshttp://www.thegeekstuff.com/2008/06/how ... nagios-30/http://techchorus.net/how-install-and-c ... ios-centos
And some Android apps that can tap into Nagios: http://www.androidpolice.com/2010/06/06 ... onitoring/
CRUCIAL: When entering the URL in the Android app, use https://domain.tld/nagios/cgi-bin
and NOT /cgi-bin/nagios and NOT just /nagios