Vii Studio

Vii Related Posts gives you a widget to output lists of posts related to the current post. You get to decide how the posts are related (random, categories, tags,…) and a whole lot more.
A ViiStudio plugin.
<h4>Features</h4>
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Speed 
Unlike other related posts plugins, Vii Related Posts is super fast. It doesn’t run resource-intensive algorithms on your database and it doesn’t bog down your server. If you’re looking for a plugin that’ll run through all your content and match related posts word-for-word, there are plenty of good ones out there. If you’re looking for a simple, lightning-fast related posts plugin, you’re in the right place.
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<li>
Display related posts as post thumbnails, links or excerpts.
</li>
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<h4>Settings</h4>
You can easily configure Vii Related Posts with the following settings. These settings are directly configurable in each widget.
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<li>
Title 
The title of your widget. Give it blank when you need to hide the widget title.
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Number of posts to show 
The total number of posts you would like to show in each section.
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Related By 
Choose the method by which you want to show related posts. You can choose random, lasted posts, popular posts (by comments), post have same tag, or post have following tag. This field automatically populates with all available possibilities.
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Tag (optional) 
When you choose related by post have following tag, specify custom tag in this field.
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Vii Related Posts

Reduce bounce rates by display related (same category, same tag, specify tag), lasted, popular (comment count) or random posts to your site in the sim …

jaric

Basing on WordPress Plugin “tagthepress” .It using for tag suggestions by Yahoo cas API 
http://tw.developer.yahoo.com/cas/
With this plugin, you will be able to use the the Web service for your posts. Just click on <code>Fetch tags</code> and the title, content and optional excerpt of your post will be analyzed by Yahoo cas API in order to find words that could be useful as tags.
It currently works with English, Chinese

AutoTags

Basing on Wordpress Plugin "tagthepress" .It using for tag suggestions by Yahoo cas API

Vii Related Posts vs AutoTags