The Yoast SEO Premium plugin (formerly known as WordPress SEO by Yoast Premium) adds several features to the Yoast SEO plugin. This plugin is written from the ground up by Joost de Valk and his team at Yoast to improve your sites SEO on all needed aspects. While this WordPress SEO plugin goes the extra mile to take care of all the technical optimization, more on that below, it first and foremost helps you write better content. WordPress SEO forces you to choose a focus keyword when youre writing your articles, and then makes sure you use that focus keyword everywhere.
Product page: http://yoast.com/wordpress/plugins/