There are literally dozens of add-on projects to enhance and extend Nagios even more. For a very updated list of what is available, I highly recommend visiting NagiosExchange.org.
* Apan
* PerfParse
* Cacti Plugin
* Nagvis
This list is still being compiled.
The number of projects that extend and enhance Nagios is staggering. If you've read the list above and have a need for something not listed, try searching the following sites for Nagios add-ons and enhancements. New projects appear every day.
* NagiosExchange.org
* SourceForge