Lorna Timbah

Tag My Hash (#TagMyHash) is a lightweight WordPress plugin that bridges social-media-style hashtag usage with WordPress’s native tag taxonomy. This plugin scans your post content, identifies anything starting with a pound (a.k.a. hashtags sign #), and turns it into a functional part of your site’s tag structure. You no longer have to manually scroll down to the “Tags” meta box every time you want to categorise a quick update. A live front-end demo of this plugin can be found @ <a href="https://lornatimbah.com" rel="nofollow ugc">LornaTimbah.com</a>.
Features:
v.1.0: 
– Automatic hashtag detection in post content 
– Auto-create missing WordPress tags 
– Auto-assign hashtags as tags on save 
– Convert hashtags into clickable links to tag archive pages 
– Simple settings page (enable/disable + mode toggle) 
– Standard Posts support

Tag My Hash

Automatically converts #hashtags in your posts into WordPress tags and clickable links to tag archive pages.

truongthanhtungitvn

If your site has a large number of post which are not placed in categories or tags, This plugin will help you do it automatically. 
You just need to define keywords for earch categories/tags and do it schedule by cronjob or manually by click “Action now”.
<ul>
<li>Set target keyworks for each category/tag</li>
<li>Set rule for the litmit of category/tag for each post</li>
<li>Set cronjob for auto category/tag by minutes, hours, days.</li>
</ul>

Daknetcorp Auto Tag Category

Daknetcorp-auto-tag-category allows you auto tag/category you post by target keywords in the post's content.

Tag My Hash vs Daknetcorp Auto Tag Category