Here is the list of few best Wordpress plugins which will help webmasters to improve there ranking in Google, and such other search engines. All plugins are free and supported by there developers with very frequent updates according to needs.
1-All in One SEO Pack
Automatically optimizes your Wordpress blog for Search Engines (Search Engine Optimization).
2-Platinum SEO Pack
Platinum SEO Plugin offers Complete onsite SEO solution for your Wordpress blog.Platinum SEO Plugin offers all the functionalities of All In One SEO
3-Redirection
Redirection is a WordPress plugin to manage 301 redirections and keep track of 404 errors without requiring knowledge of Apache .htaccess files.
4-HeadSpace2 SEO
Controls almost every aspect of your site’s meta-data, including advanced tagging, Analytics, and dozens of plugins. The best WordPress SEO solution!
5-Advanced Permalinks
Allows multiple permalink structures and category-specific permalinks without needing redirects.
6-AskApache Google 404
Displays unbeatable information to site visitors arriving at a non-existant page (from a bad link). Major SEO with Google AJAX, Google 404 Helpe
7-SEO Friendly Images
SEO Friendly Images automatically adds alt and title attributes to all your images. Improves traffic from search results and makes the tags w3c/xHTML
8-SEO Smart Links
SEO Smart Links provides automatic SEO benefits for your site in addition to custom keyword lists, nofollow and much more.
9-Permalink Redirect
Redirects all crap away from the end of the URL.
10-SEO Title Tag
Search engine optimize your blog’s title tags. Mass edit the title tags of posts, pages, category pages, tag pages – indeed, any URL!
11-Robots Meta
This plugin makes it easy to add the appropriate meta robots tags to your pages, disable unused archives, nofollow unnecessary links as well
12-Another Wordpress Meta Plugin
Add META tags like keywords and descriptions to your posts and pages.
13-KB Robots.txt
Edit your robots.txt from within Wordpress to control search engine activities.
14-Optimal Title
Optimal Title mirrors the function of wp_title() exactly, but moves the position of the ’separator’ to after the title rather than before.
These are great seo plugins, l use the third one.