WhosOn is a centralized logon accounting system built upon SOAP web services and SQL for storage. The client application is simply runned thru logon/logoff hooks to send logon events to the server side for usage accounting.
You need an web server for hosting the web service. Use IIS 5.0/6.0 for installing the ASP.NET web service or Apache to install the PHP web service. The web service requires access to an SQL database.
The client is intended to be runned from an logon/logoff hook to register logon events thru the SOAP service. For Windows clients, this is typical done using GPO (group policy objects) in the domain policy.
The same client can be used to list collected logon data using its quite advanced filtering capabilities, thus allowing integration with other system services.