[Nagios® | http://www.nagios.org/] 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] OperatingSystem, but works fine under most [Unix] 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], [IM], [SMS], etc.). Current status information, historical logs, and reports can all be accessed via a WebBrowser. 

  

  

 * Monitoring of network services ([SMTP], [POP3], [HTTP], [NNTP], Ping, etc.) 

 * Monitoring of host resources ([CPU] load, disk and memory usage, running processes, log files, etc.) 

 * Simple plugin design that allows users to easily develop their own host and service checks