I have a little experience with server monitoring products from my previous job (mainly Nagios), but never had the chance to set up a monitoring scheme from scratch. Every time there was a number of machines that needed to be monitored, running the Nagios Daemon (I think there is a fancy name for this, but it eludes my memory at this point), and a dedicated machine running Nagios.
Now that I am self-employed, most of the time, I find myself setting up a single server for my clients (storage, mail, WLAN controller, etc...).
Is there some kind of LIGHTWEIGHT monitoring solution that can be run on the same machine that it is supposed to be monitoring? I am well aware that Nagios (and the other commercial and open-source monitoring applications) are capable of doing this but I object on the grounds of -one- it being overkill (not to mention misuse of system resources) and -two- being bad practice doing so.
Please feel free to suggest any solution, and of course educate me in case I am being erroneous about something. Let it be noted that I am open to ideas that might require a degree of "hacking" on my part. I am not explicitly set on deploying a pre-built app, any solution will and can be considered.
EDIT: There is a single requirement for this. I need it to be able to respond in a custom way to events (assigning a script as a response to a trigger will do).