We nearly forgot to post this: We released our first free plugin last week. It’s really simple: it just makes it so a user can only put a post into a single category. You can install it by downloading it from the WordPress plugin repository or just by searching “Only One Category” on the Plugins > Add New page inside your WordPress dashboard.
Why would you want to use this plugin? Because if you have either functional or aesthetic reasons each post on your site should be in only one category, the default checkboxes don’t communicate that well to you or your site’s other contributors. So we just turn those checkboxes into radio buttons, which only let you select one. There’s a little more about the plugin on our page for Only One Category.