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.

BIOSTALL

Selecting tags each time you post can be tedious. You also have to have an awareness of all the tags you’ve added in the past.
This plugin adds tags for you as you type. It looks at all the tags you’ve used in the past and, if it finds the same word(s) in your content, adds the tag automatically.

Automatic Tag Selector

Nice

Selecting tags each time you post can be tedious. You also have to have an awareness of all the tags you've added in the past.

Tag My Hash vs Automatic Tag Selector