In the past I've posted about Scribe SEO plugin for WordPress (and now Drupal, etc). It has added functionality to not only go through a checklist for best SEO practices (word count, meta data, keywords, links, ) but now it will help you do keyword research from within WordPress. It's a cool tool. Currently I have 300 evaluations per month. When I get ready to post, I click a button and Scribe SEO evaluates my post. It runs me $27 a month. If I do not use all of my evaluations, I lose them at the end of the month.
I was told today about SEOPressor. This plugin has similar functions to help you with your search engine optimization. It goes through a checklist (Do you have keywords in your H1, H2, H3 headings), it checks your keyword density, is does an analysis of your images and links, it bolds your keywords, it evaluates your post and offers suggestions. The price is $47 for a single website or $97 for unlimited sites.
I know as time goes on, I get bummed when I pay Scribe for evaluations I never used, and it would be nice to have a one time payment options for Scribe (my guess is he is using some service to check keywords that is not free).
If you are new to SEO, or if you are using virtual assistants, this may be something you look into as both plugins help you tweak your copy to maximize your posts and get top 10 Google rankings.
Check it out at seopressor.com