Calendar Event subscribing
 

There is a beautiful mod written for the 3.7 version of VB and called Farecaster Event Attendace that let every registered user to book his presence to an event inserted in the calendar. i think that this feature should be included in vbulletin (optional or not* you decide) since many VB users use the calendar to notify event to theirs members and to give the possibility to every member to attend and having the event manager know how many people will attend* should be a must...