jfoucher

This plugin uses the yahoo.com API to find the most relevant keywords from the content of your post, and then adds them as tags.
The tagging is fully automatic, so if you’re using a plugin like feedwordpress to display RSS feeds on your blog as posts, everything will get done as the feed posts are published. No user intervention necessary!

Auto-tags

Automatically add relevant tags to new posts.

kamiel79

Adding tags on the fly by just prepending a word you want to use as a tag with a “#” was a functionality I missed in the WordPress backend.

Tag With Hash

I'll give 5 rating if this plugin updated and works

Not works

Automatically turn terms into tags if you precede them with a #. Nonexisting tags are created.

Auto-tags vs Tag With Hash