From collectd Wiki

Nagios is a free, open source monitoring tool. collectd can interact with Nagios in a number of ways:

• collectd-nagios, a tool that can export collectd data in a Nagios-compatible format. See the collectd-nagios(1) manpage for details.
• exec-nagios.px, a script for the Exec plugin that can be used to execute Nagios checks and import “performance data”. See the inline documentation in contrib/exec-nagios.px for more information.