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

Birchdunford

Tags, tagging, and post tags made easy. Stop thinking about what tags you should add, forgetting to add certain tags, and typing in tags by hand. Let Tagbag do the work for you. Just look at the list of suggestions, click the ones you want, and move on. More on Tagbag: 
<a href="http://www.explodybits.com/2012/04/tagbag-wordpress-plugin/" title="explodybits (xb)" rel="nofollow ugc">explodybits (xb)</a>

Tagbag

Tagbag, takes the thought out of selecting tags for your post

WP Keyword Suggest vs Tagbag