Nagios: Another Monitoring Tool
I am going bonkers over server, process and network monitoring tools today. I guess it is because I often find it frustrating when services go down during the holidays. I have experienced a couple of problems before, when I was still the Director of the University Computer Center of the University of the Philippines. During my term, we had problems about the network going down due to power interruptions — one of our uplink was housed in one of the colleges in the university and the college admin decided to turn the power off for the holiday. 
Anyway, we've transferred that off of the building into somewhere we have access and control.
Anyway, like the tools that I have mentioned before, Nagios is another option, albeit more popular than Munin. Like Munin, it also supports plug-ins. Nagios also has the feature to alert administrators when there is something wrong with the services, server or network, either via e-mail, pager or even SMS. It even has a WAP interface. Nice, eh?
So if you are a system/network administrator, you have plenty of options to monitor your services.