John C. Welch has written up a thorough walk-through on installing Nagios, "an open source host, service and network monitoring program" (shamlessly lifted from their website).
I haven't tried this, as I have no need for it (nor do I have OS X Server), but if you'd like a network monitoring tool on your Server box, this seems like a good one (check their stats page for some data on who's using their tools).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041025052113531