Nagios is a host and service monitor designed to inform you of network
problems before your clients, end-users or managers do. The monitoring
daemon runs intermittent checks on hosts and services you specify
using external "plugins" which return status information to Nagios.
When problems are encountered, the daemon can send notifications out
to administrative contacts in a variety of different ways (email,
instant message, SMS, etc.). Current status information, historical
logs, and reports can all be accessed via a web browser.

WWW: http://www.nagios.org/