NRPE
NRPE is an addon that allows you to execute plugins on remote Linux/Unix hosts. This is useful if you need to monitor local resources/attributes like disk usage, CPU load, memory usage, etc. on a remote host. Similiar functionality can be accomplished by using the check_by_ssh plugin, although it can impose a higher CPU load on the monitoring machine - especially if you are monitoring hundreds or thousands of hosts.
NSCA
NSCA is an addon that allows you to send passive check results from remote Linux/Unix hosts to the Nagios daemon running on the monitoring server. This is very useful in distributed and redundant/failover monitoring setups.
NSClient++
Monitoring private services or attributes of a Windows machine requires that you install an agent on it. This agent acts as a proxy between the Nagios plugin that does the monitoring and the actual service or attribute of the Windows machine. Without installing an agent on the Windows box, Nagios would be unable to monitor private services or attributes of the Windows box.
NDOUtils
NDOUtils is an addon that allows you to store all status information from Nagios in a MySQL database. Multiple instances of Nagios can all store their information in a central database for centralized reporting. This will likely serve as the basis for a new PHP-based web interface for Nagios in the future.
