I wrote a couple SNMP plugins for Nagios to check disk space and running processes. They are now in the contrib directory in the Nagios Plugins package.
They both should work on Windows 2000 (and up) and Novell. HP-UX, unfortunately, does not ship with the Host-Resource MIB by default, so I'm using the Net-SNMP daemon at work. It's a little flakey, but gets the job done. SNMP Research provides an extended SNMP daemon for HP-UX and other operating systems that includes the host resource MIB.

I'm also author and maintainer of the Nagios::Object and Nagios::Cmd modules on CPAN.