What tool do you use to monitor your servers?
Can you tell which monitoring software runs on your linux boxes?
I know about Zabbix in my opinion looks quite well, but I'm opened to hear your hints.
Nagios hands down ... we also use Munin which does performance monitoring.
We are currently using Zenoss, which is free + opensource. Installation was easy, setup was a little harder- we are only using the basic features, but there are many great addon's that people have written as well.