Nagios : Nagios is the Network Monitoring Tool , It watches hosts and services that we specify,
alerting us when things go bad and when they get better.It is mainly used by the large company to monitoring the infrastructue of the company. It help the Infrastrucre team to monitior the memory,cpu etc usage of the network elements and if any issue occures in the network , Network team can over come or resoved the issue before it hamper the production. There are so many in-build plugins available which help the nagios admin to monitor all type of the network elements or servers (window/unix). It support all the UNIX flavor OS.