CustomFields is a plugin that allows you to define custom fields that will appear on the entry editing screen and author profile screens. This allows you to store far more things about an entry or author for example you can now create complex author profiles.
Now, I’ve complained about custom fields before in general, but this lets the fields be defined per blog. With this, you can do a lot. (And this is a step up from my own user-defined fields hack, since that hack made the fields global to the installation.)
Going back the the “sermons” example I tend to use a lot, you could have a blog called “Sermons” with all the custom fields in there that you need (length, topic, biblical reference, etc). Other blogs store other objects and have other fields.
This plugin has the additional functionality of treating authors as an archive type, so you can create author archives with detailed information and a collection of blog postings about authors. This is long overdue.