This is as much for my own edification and future self as anything else. Three system / health monitoring tools of which I'm aware are:
Nagios and the unfortunately-named but always excellent companion nagiosexchange.
Lighter-weight tools are monit and mon. A friend of mine tried the latter and proclaimed it Good, although the names for both of those are pretty crap, especially mon. At least give it a cute tagline or something. Software authors should always google the names they're considering; think of somebody who sort of remembers the name but maybe not quite, but wants to find it again. If the mighty GOOG returns 10,000 hits, maybe your name isn't so good after all.