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

Casey Bisson

Integrates <a href="http://www.opencalais.com" rel="nofollow ugc">OpenCalais</a> tag suggestions in the WordPress edit interface. Supports multiple and custom taxonomies. Maps between <a href="http://www.opencalais.com/documentation/calais-web-service-api/api-metadata/entity-index-and-definitions" rel="nofollow ugc">OpenCalais entities and facts</a> and WordPress taxonomies, including custom taxonomies.
Suggested tags are shown in the editor interface in the same UI sections where tags are typically added.
Use of this plugin requires an API key and is limited by the API license and terms of service. Register for an API key at http://www.opencalais.com/user/register .
See installation section for custom configuration instructions.
<h4>Batch auto-tagging</h4>
The plugin also includes (not pretty) tools for automatically tagging previously published content. Logged-in users with sufficient permissions can visit <code>wp-admin/admin-ajax.php?action=go_opencalais_autotag</code> which will automatically start tagging content.
<h4>In the WordPress.org plugin repo</h4>
Eventually at: https://wordpress.org/plugins/go-opencalais/
<h4>Fork me!</h4>
This plugin is on Github: https://github.com/GigaOM/go-opencalais

Gigaom OpenCalais

Easier tagging thanks to suggestions from OpenCalais

WP Keyword Suggest vs Gigaom OpenCalais