Nagios is an open source host and service monitoring application designed to run under the Linux operating system, but also works on other platforms as well. Key features: Nagios, built out of the project formerly known as NetSaint, monitors network services such as SMTP, POP3, HTTP, NNTP and more. The software also monitors host resources such as processor load, disk and memory usage, running processes and log files. It can incorporate environmental factors such as temperature. The open source application can also be customized by network managers via plugins.