Monitor Whatever: a tiny Firefox and Thunderbird extension called whatmon, which can monitor almost anything you wish to monitor. Number of logged on users? Current load average? Health of your LDAP servers? Mail server queues? No problem for whatmon, as long as you can create a CGI(Common Gateway Interface) program in Perl, C, or any other language you are comfortable with, or even a PHP or Active-Whatever script which runs on your web server and can produce a wee bit o' XML(eXtensible Markup Language).
The XML contains an integer status and a single line of text. In practice, that line of text can contain whatever the administrator desires.
The program that produces that line of text also returns a numeric integer indicating okay, warning or critical so that the extension can colour-code the status bar accordingly.
The monitoring extension requires two preferences to be set. I've named these whatmon.url and whatmon.refresh. They are the URL from which the XML is to be read and the frequency in seconds in which the extension should do a GET request from the URL.