Top Cat is a WordPress plugin that allows you to specify a main category for your posts. Even though WordPress posts can have more than one category, you may want to specify which of them is the main category. Top Cat lets you do this when creating or editing posts, and also provides template tags for displaying the main category of each post.
Using style sheets, you can then apply different styling to posts based on their main category, even if they belong to the same set of categories. For an example, see my Bennettarium website.
For details, examples, download and so on, see the main Top Cat WordPress plugin page.