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.

daxiawp

Automatically search for custom tag list, if the content of the article contains the text automatically add the article tags
自动搜索自定义的标签列表，如果文章内容包含该文本，则自动添加文章标签。 
详情：<a href="http://www.daxiawp.com/dx-auto-tags.html" rel="nofollow ugc">http://www.daxiawp.com/dx-auto-tags.html</a>
<h3>Arbitrary section 1</h3>

DX-auto-tags

自动搜索自定义的标签列表，如果文章内容包含该文本，则自动添加文章标签.

Tag My Hash vs DX-auto-tags