For each post, author can choose which category is used in permalink.
WordPress 3.0 comes with a new filter that lets us customize what is used for each permalink structure tag, other than WordPress default.
One of these permalink structure tags is
%category%. By default, WordPress always use the category with lowest ID, making
%category% impractical for SEO optimization.
Hikari Category Permalink allows post authors to choose among each post's categories, which of them is used in that post permalink, giving much more flexibility and power to permalinks.
This plugin is a fork of Dmytro's sCategory Permalink. It has all original features and is compatible with original options, while being more stable and simple, and also fixes 2 recurring bugs.