The UserMonitor Project was sponsored by Cogent Innovators, LLC, and has been released under the terms of Version 2 of the GNU General Public License.
This program runs as a service under Windows 2000 and up (or as a background process for Windows 98/ME) and keeps track of which users are logged on to the computer. It reports that list along with the user's domain and computer IP addresses to a MySQL database of your choosing.
It can also log to a central syslog server. It was developed for use with the Squid proxy server due to limitations in proxy logins. The lowest-credentialed user is being used to restrict all internet access to the machine.