Just completed a joomla component that is based on the Digg idea. It allows the posting of content which is then voted on by users. The postings can be voted upwards or downwards and marked as spam. There are 2 levels are categories for the postings to be categorized.
In the admin section the admin user has a configuration section where they can setup automatic levels for the postings to appear on the home page or for postings to be removed (deactivated) if a certain level of spam votes have been reached. The admin user also has the ability to manual manage the postings and setup the categories and subcategories.
I made use of Ajax to handle the the Category/Subcategory relationship in the form adding/editing a posting.