My monitoring tool: WebInject is a popular plugin in the Nagios community. A Nagios user (Felipe Ferreira) just wrote a small how-to on setting up WebInject. (though the official manual is much more thorough)
Webinject How to
"When monitoring websites, many times we will need more than just checking if the site is up. We may need to see if the internals of the website are working. A good example is making a user login check. Using Nagios alone it can not be done, but thanks to Corey Goldberg, it is possible using his script webinject.pl. [snip...] And yes it works with HTTPS and redirections."