Rob Saurini

If you have more than one category associated with a post, how do you tell which category takes precedence? This plugin gives you the ability to promote the category to the main one on the post edit page.
This gets stored in post meta for later use. Soon, I will add some simple functions that will allow you to get the main category on the front end.
Use
<ul>
<li>You can use the shortcode [rs_print_main] inside the post to print out the category title.</li>
<li>In the theme, you can use rs_get_main_category_id( $post_id ) to return (not print) the id of the main category.</li>
</ul>

RS Simple Category Selector

Gives you the ability to promote categories associated with a post.

Marios Alexandrou

Ultimate Category Excluder, abbreviated as UCE, is a WordPress plugin that allows you to quickly and easily exclude categories from your front page, archives, feeds, and searches. Just select which categories you want to be excluded, and UCE does all the work for you!

Ultimate Category Excluder

Ultimate Category Excluder allows you to quickly and easily exclude categories from your front page, archives, feeds, and search results.

RS Simple Category Selector vs Ultimate Category Excluder