I spend a good part of any day in my favorite web browser, and even if I don't see its whole window, the lower right part of the status bar is always visible under a pile of other application windows (mostly SSH sessions). That status bar is the ideal place to put up a small monitor with which I can keep an eye on a number of important servers, without having to wait for alerts sent to me by Nagios.
To this effect I have created 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). There is one thing that whatmon can't watch: the web server from which it retrieves that XML ... :-)