Nagios
Archived
Written in November 2010. Kept as it was, so versions, links and details may be out of date.
A good tutuorial on using service checks by SSH.
wiki.nagios.org
Useful commands
Verify config
/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
Plugin directory
/usr/local/nagios/libexec
Commands directory
/usr/local/nagios/etc/objects