On this somewhat binary day (101111), I just submitted my first Munin plugin to http://exchange.munin-monitoring.org and once it is approved it should be available at http://exchange.munin-monitoring.org/plugins/system_users/details/ .

It simply lets you graph the number of logged in users with Munin.
Example of the daily graph:

For an explanation of what munin is have a look here: http://munin-monitoring.org/
From the site:

Munin is a networked resource monitoring tool that can help analyze resource trends and “what just happened to kill our performance?” problems. It is designed to be very plug and play. A default installation provides a lot of graphs with almost no work.