netmon
Nagios is a host and service monitor designed to inform you
of network problems before your clients, end-users or managers do. It
has been designed to run under the Linux operating system, but works
fine under most *NIX variants as well. 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.