Also consider GroundWork, which unifies a bunch of open source
monitoring tools, including Nagios(R), making it easier to deploy and
manage.  For example: GroundWork puts the nagios configuration into a
MySQL table.

You can download GroundWork from SourceForge at:

http://sourceforge.net/projects/gwmos/

There's even a CentOS-based bootable ISO to try it without installing
it.