No Category Base (WPML)
This plugin removes the mandatory 'Category Base' from your category permalinks. It's compatible with WPML.
Plugin Tag: category slug
WP Remove Category Base
Removes the category base slug from the category archive permalinks (URL's).
WP Remove Category prefix
Removes the category base slug from the category archive permalinks (URL's).
Primary Cat
Set a primary category for your posts… and then do stuff with it. [primary-cat]