Big brother was a life saver for me a few times on small projects as a temporary measure. Xymon is the modern fork and is still maintained.

Another one that I really liked and even have this crazy idea of reviving as a side project mayhaps is Argus (tcp4me)... it was written in perl and was my main intro to the beautiful hell that is perl. These days though between sensu, prometheus, zabbix, and nagios, we really have plenty of good monitoring options.