Linux.com: Getting Started with Nagios 2.1

“Nagios, the premier open source network monitoring program, is
celebrating its 2.1 release. With four years of development under
its belt, Nagios is powerful enough to replace expensive
proprietary monitoring products and become a tool your organization
can’t live without.

“The Nagios change log shows more than 126 improvements since
version 1.3, which came out a year ago…”