Nagios is a system and network monitoring application. It watches hosts and services that you specify, alerting you when things go bad and when they get better. Some of its many features include monitoring of network services (SMTP, POP3, HTTP, NNTP, ICMP, etc.), monitoring of host resources (processor load, disk usage, etc.), and contact notifications when service or host problems occur and get resolved (via email, pager, or user-defined method).
For downloads and more information,
visit the Nagios homepage.
Nagios is by far the best network monitoring tool. You can extend it to monitor anything you want, with tons of plugins already written. One of the best open source projects ever!
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