robfelty

Category reminder uses javascript to assert that a category has been assigned 
to a post before publishing it. If no category is assigned, then it will 
prompt the user to assign before publishing.

Category Reminder

This plugin requires users to assign a category to each post other than "Uncategorized".

pressupinc

davidbhayes

Only One Category makes sure users only select one category for their post by making the selectors radio buttons, which only allow one category to be selected.
Rather than spend all your time maintaining the editorial style of only putting posts into a single category, this plugin is a quick way for you to enforce that standard. It makes it impossible for a person to accidentally break your site’s layout of functionality by putting a post in multiple categories. They just can’t do it, whether by accident or intention.

Only One Category

Works great

Limits a post to a single category by changing the checkboxes into radio buttons. Simple.

Category Reminder vs Only One Category