With out a doubt the most useful plugin I’ve found for search engine optimization is the WordPress SEO plugin by Yoast. This is a great tool that keeps you aware of all the pertinent information while you are posting.
About the Yoast plugin:
Checks the length of your description and other meta data.
Helps your posts by optimizing by a keyword of your choice.
Keyword density tool.
Meta data tool.
keyword research options.
The free version of the WordPress plugin offers a lot of SEO tools right out of the box or you can sign up for a subscription basis which will not only offer more marketing opportunities, but also get you in contact with the Yoast staff.