Nagios -
http://www.nagios.org/
Formerly known as SAINT. OSS, so it's free.
Main features:
* Monitoring of network services (SMTP, POP3, HTTP, NNTP, PING, etc.)
* Monitoring of host resources (processor load, disk and memory usage, running processes, log files, etc.)
* Monitoring of environmental factors such as temperature
* Simple plugin design that allows users to easily develop their own host and service checks
* Ability to define network host hierarchy, allowing detection of and distinction between hosts that are down and those that are unreachable
* Contact notifications when service or host problems occur and get resolved (via email, pager, or other user-defined method)
* Optional escalation of host and service notifications to different contact groups
* Ability to define event handlers to be run during service or host events for proactive problem resolution
* Support for implementing redundant and distributed monitoring servers
* External command interface that allows on-the-fly modifications to be made to the monitoring and notification behavior through the use of event handlers, the web interface, and third-party applications
* Retention of host and service status across program restarts
* Scheduled downtime for supressing host and service notifications during periods of planned outages
* Ability to acknowlege problems via the web interface
* Web interface for viewing current network status, notification and problem history, log file, etc.
* Simple authorization scheme that allows you restrict what users can see and do from the web interface
