Do you want to show your users what is being said in Twitter about a certain keyword? Want to increase your keyword density for a word? This plugin does exactly that.
You can configure the keyword, Tweet’s language and the number of tweets that appear on the sidebar.

Twitter Keywords

Add tweets about a certain word, in certain language and from a specific user on your sidebar

Nicolas Marin Torres

With this plugin you will be able to take advantage of long tail keyword so you can learn more about your niche. 
From the list of keywords suggested you can put it directly in the title of your article or add it as a tag. It is very easy to use, from the same page to add item, it shows the widget in the left menu. 
This plugin helps you generate ideas for your post.
For example if you want to write a post about ‘Keyword Reasearch Tool’, these are tips that shows you:
<ul>
<li>
Keyword research tool
</li>
<li>
Free keyword research tools
</li>
<li>
Keyword research tool review
</li>
<li>
Google keyword research tools
</li>
<li>
Keyword research tool online
</li>
<li>
Keyword research seo tool
</li>
<li>
Bing keyword research tool
</li>
<li>
Keyword research tools 2013
</li>
<li>
Keyword research tools list
</li>
<li>
Free online keyword research tool
</li>
</ul>
This plugin makes to calls to seowp.es, the first one to create the URLs of the autocomplete service, and the second one to filter and complete the results. Does not require registration.
<a href="http://www.youtube.com/watch?v=vO64yHrHknA" title="YouTube" rel="nofollow ugc">Watch a demo</a>

WP Keyword Suggest

This SEO plugin offers keyword suggestions, taken from autocomplete google, yahoo, bing... up to 250 keywords ideas

Twitter Keywords vs WP Keyword Suggest