See http://ultimatemarketingstrategies.net for the full article on comparing three on-page SEO WordPress.
I go through the details of:
1. SEOPressor – the most well known of the onpage SEO plugins, created by Daniel Tan.
2. BloggerHigh SEO – A free clone of SEOPressor.
3. ClickBump SEO – A new onpage SEO plugin created by Scott Blanchard.
Find out the differences between these plugins and which plugin will best suit you.