GroupCalendar for Outlook (No User Count license)
GroupCalendar for Outlook (since 2003) was designed with simplicity in mind. Installation is a two step process and takes only 1 minute per user. After that, all users have an extra calendar folder in outlook under the normal calendar.
The groupcalendar shows all appointments of all users together in one calendar. Scheduling an appointment for another user simply means creating that item directly in the groupcalendar and the software will give a list of users to assign the new item to.