Wow.. I love seeing my Open Source code propagate through the community.
I am the author of WebInject, which is a popular web application/service test tool that also works as a Nagios plugin. GroundWork has an Open Source monitoring system based on Nagios.
I just stumbled across an awesome screencast tutorial from GroundWork called "Using WebInject To Monitor Web Applications And Services". It shows how to install and setup WebInject as a monitoring agent.
Go check it out!
you can also download the screencast and PDF without registering by going here: