I've used nagios and before that, ganglia for some monitoring. I've been looking into cacti lately for monitoring qmail message status and an asterisk installation for the number of active channels.

Many of these systems use rrdtool and even though I've used that tool directly before, with Cacti at least its a bit tricky to get familiar with the way the gui and the different templates are interconnected. The look and feel sure does seem polished though.