Customizing Admin

Hi. Is there a plug-in for say, adding a new input form to the admin interface?

I would like to add a tab ‘Event’ to the Write interface. This would edit very much like the Posts and Pages, except it would have start-date and end-date fields that could then be used for the presentation logic.

I have seen various calendar plug-ins, but I can’t grasp how they can possibly work without dates. Since I might like to add other custom fields (corresponding to additional columns in the database) it would be good to know the easiest way, generally, to futz with wp-admin.

Something like Plone Archetypes would be ideal. Is there anything like that around?