Are you one of those special geeks that think there is never TMI (too much information)? If that describes you then Nagios is the monitor for you. Once installed, Nagios will keep you busy with more information about your system than you ever thought possible. More here

You want features? Nagios is full of them. Nagios can monitor your entire network, you can handle problem remediation, you can plan network downtime, you can watch hosts, systems, services, applications..there’s very little you can’t do with Nagios.