SEO WordPress Plugins
Free 
All in One Schema.org Rich Snippets
Use search rich microfomrats snippets from Schema.org to add a new dimension to search engine results. Customize the output of the rich snippets, and add to any post with a simple metabox.
Freemium 
Google Analytics for WordPress
Go beyond the basic set-up of Google Analtyics and track custom variables, and chose which links to track as internal and external, so that analytics in the Google dashboard contain more information.
Free 
Jetpack
The swiss army knife of plugins, the Jetpack plugin comes with a few dozen features to enhance your WordPress site, from carousels and comments to a full JSON REST API.
Freemium 
WordPress SEO by Yoast
Access tools that increase your presence in search engines. Edit titles, generate XML sitemaps and edit .htaccess and robots.txt files.