I’ve just released the <MTExtendedAttribute> plugin for Movable Type into the wild. I’ve been working on this off-and-on for the last month or so, but other things have been keeping me busy, so it’s taken a while to get it polished enough for anyone else to use.
Movable Type only gives you a limited number of fields in which to store data for entries, blogs, categories and authors. With <MTExtendedAttribute>, you can add new fields by adding the extra data into a CSV file, which you drop into your file system, or into a Template Module. I’m using it right now to define multiple icons for each category in my “Quick Reviews” sidebar (which is itself a Movable Type blog).
It’s possible/probably that future versions of MT will allow you to defined custom fields for your entries, categories, and other stuff, but this plugin allows you to do this right away.