The Primary Category extension allows you to assign a primary category to an entry in ExpressionEngine with only one extra click. When you view a front-end page and have a weblog entries loop you can pull the primary category variables like all native EE category variables. Using the exp:primary_category loop, you can pull the same variables, so you have more flexibility. You can also have Primary Category remember the categories and primary categories you assign to entries to apply to the next entry, as well as configure the extension to require a Primary Category on a per Weblog basis. Additionally, your regular categories list is organized in a neat and tidy, controlled manner.