I'm looking for a plug-in or product that will act as a remote probe and perform tests then send back the results to the central Nagios server.

Reason for this is that I'd like to monitor internal systems and servers at customers, but don't want to allow all the traffic passing the firewalls. Ideally I'd like a soft-probe that would be installed and then perform the tests and send back the results (via SSH) to the central Nagios installation.

Does anyone know of a product or plug-in that would offer such service? If not Nagios, is there any other monitoring system that does such a thing (ideally open-source)?